Sulfur (S)

Characteristics of Sulfur

Non-metal

Atomic number – 16

Atomic mass - 32.065

Number of isotopes – 25 isotopes:

4 of which are stable isotopes (95% - 32 with 16 neutrons, the remaining

5% - 33 with 17 neutrons, 34 with 18 neutrons, 36 with 20 neutrons)

21 of which are unstable radioactive isotopes but are short lived (seconds

to days).

Sulfur is the tenth most abundant element in the universe. It is a pale yellow,

tasteless, odorless and brittle material that is non-toxic. Sulfur is solid at room

temperature, but melts easily at a temperature of only 239o - slightly above the

boiling point of water (212o). Sulfur does not dissolve in water (insoluble) and

forms stable compounds with all elements except the noble gases.

As compared with oxygen, which can form covalent bonds to one, two or exceptionally three other

atoms, the most striking feature of sulfur is its added ability to bond to four, five or six atoms.

Sulfur burns if heated and is referred to as brimstone in ancient literature – “fire and brimstone”.

In nature sulfur can primarily be found occurring in three forms 1) elemental sulfur, 2) sulfides, and 3)

sulfates. Together these three forms of sulfur constitutes about 0.05 percent of the Earth’s crust. After

oxygen and silicon, sulfur is the most abundant constituent of minerals.

Natural Sources of Sulfur

Elemental sulfur is a natural component of the

environment found in soil nearly everywhere in the

world. There are geological areas where elemental sulfur

occurs as massive sedimentary deposits. Examples of

this occur in areas near hot springs and volcanic regions.

Significant deposits of elemental sulfur also exist in salt

domes along the coastal regions of Texas and Louisiana.

In areas of heavy deposits sulfur is extracted from the

ground using a process called the Frasch process. Because the boiling point of sulfur is low, the Frasch

process involves pumping down superheated steam down into the deposits, causing the sulfur to liquefy.

The liquefied or molten sulfur is then removed by pumping air down into the deposits, causing it to rise

to the surface. Once on the surface the molten sulfur once again solidifies. The Frasch process produces

sulfur of very high purity of 99%.

This process is used when naturally occurring large deposits of elemental sulfur are found. However,

most of the Earth’s sulfur is found in underground deposits “tied up” as rocks, common minerals and

salts or buried deep in the ocean in oceanic sediments.

Some materials containing sulfur with

their percentage content are:

Material Wt. %

Earth’s crust 0.052

Soil 0.01 - 0.05

Coal 1-14

Oil 0.1 – 14

Natural Gas 0.1 – 40

Gypsum 18.6

Because sulfur is found naturally in the Earth’s soil it therefore is also found naturally in plants and

animals. Sulfur in the soil, usually in the form of sulfate, is taken up and assimilated by plants. The

sulfate (SO42-) is first taken up by the plant’s roots, then through a series of enzymatic processes the

plant reduces it to sulfite (SO32-), then to sulfide (S2-) and finally the sulfur is incorporated into an organic

sulfur compound. These organic sulfur compounds are usually proteins consisting of the sulfur amino

acids – cysteine, and methionine. Animals or livestock obtain sulfur by simply ingesting plants.

Some foods containing sulfur with their percentage content are:

Food Wt. %

Cabbage 0.98

Alfalfa 0.5

Oats 0.41

Barley 0.30

Apple 0.45

Beef 0.1

Egg Whites 1.6

In the atmosphere, sulfur can be found as part of a variety of sulfur compounds (i.e. sulfur dioxide,

hydrogen sulfide) that originate from natural processes. Hot springs and steam vents, such as those in

Yellowstone National Park are examples where emissions of sulfur compounds are found. Other

examples are volcanic vapors and sea spray which play an important role. Even microscopic bacteria and

phytoplankton (ocean waters) get into the act by releasing sulfur compounds such as dimethyl sulfide

(C2H6S) into the atmosphere.

Man-Made Sources of Sulfur

Sulfur is one of the few elements that occur naturally as a “pure element” (other examples - gold,

copper and carbon). However, as mentioned most of the Earth’s sulfur is found tied up or bound to

other elements. One example is metal ores containing sulfur which are known as sulfides and sulfates.

In these situations mining may take place to recover not only the sulfur, but more importantly the

valuable metals. Metal ores are minerals or an aggregate of minerals from which a valuable constituent,

usually a metal, can be profitably mined or extracted (i.e. pyrite – iron sulfide). In the process of

removing the valuable metal, sulfur is also removed. This removal process takes place in processing

plants called smelters.

Sulfur is also found tied up in coal, petroleum and natural gas. These raw materials are valuable fuels for

powering and supplying energy to a country’s infrastructure. The burning of these substances takes

place on both an individual level (cars, lawnmowers, oil furnaces, etc.) and an industrial level (power

plants, refineries, smelters, etc.) which both result in the poisonous gas sulfur dioxide (SO2) being

released into the atmosphere. To reduce sulfur dioxide emissions a chemical process has been

developed to remove sulfur from petroleum and natural gas. This process called the Claus process uses a

technique called hydrodesulfuization (HDS).

HDS involves catalytic treatment (increases chemical rate) with hydrogen to convert the various sulfur

compounds present to hydrogen sulfide. The hydrogen sulfide is then separated and converted to

elemental sulfur. So much elemental sulfur is removed during this process that about 4,000,000 tons of

sulfur are recovered in the U.S. each year from natural gas, petroleum refinery gases and smelter gases.

Uses of Sulfur

Eighty-five percent of the elemental sulfur that is removed by HDS is used in the manufacture of sulfur

dioxide (SO2). The primary use of sulfur dioxide is in the production of the end product of sulfuric acid

(H2SO4), which is one of the chemical industries most important raw materials.

Another major use of elemental sulfur is in pesticides:

“Currently, sulfur is registered by EPA for use as an insecticide, fungicide and rodenticide on

several hundred foods and feed crop, ornamental, turf and residential sites. Sulfur is applied in

dust, granular or liquid form, and is an active ingredient in nearly 300 registered pesticide

products.

Sulfur has been know and used as a pesticide since very early times and has been registered for

pesticide use in the U.S. since the 1920’s.” (EPA – 1991)

Dusting sulfur, elemental sulfur in powdered form, is a

common fungicide for grapes, strawberry, many

vegetables and several other crops. It has a good

efficacy against a wide range of powdery mildew

diseases. In organic production, elemental sulfur is the

most important fungicide. It is the only fungicide used

in organically farmed apple production against the main

disease apple scab. It is also used against peach scab

and rot, peanut leaf spots, grape mildew, pecan leaf

curl, etc.

Smaller amounts of sulfur are used to vulcanize natural

rubbers (to treat rubber with heat and sulfur to harden

it and make it more durable). Sulfur is also used in the

manufacture of gunpowder, fireworks, matches,

detergents, corrosion-resistant concrete, and

photographic chemicals. In the world of personal care

products and medicines, sulfur is used in permanent

wave lotions, shampoos, skin care products, fungal

creams and in pharmaceuticals (i.e. sulfa drugs).

Sulfide (S2-)

Sulfide (S2-) is composed of a single sulfur atom (S) with two extra electrons in its valence shell. Making

sulfide an anion of sulfur with a negative two (2- ) charge. Sulfide is exceptionally basic and exists in

strongly alkaline aqueous solutions.

Natural Sources of Sulfides

Sulfide, however, does not exist in appreciable concentrations even in highly alkaline water. Rather, in

nature sulfide is found bonded to other elements (usually a metal or hydrogen ion) which are referred

to as sulfides or sulfide minerals.

Sulfide minerals are compounds containing a non-metallic sulfide anion (S2-) which is bonded to a

metallic cation (+).Though most sulfides are metallic there are couple of exceptions such as – carbon and

phosphorus which are non-metallic. Most sulfide minerals form by means of precipitation in

hydrothermal fluids or other aqueous solutions. Sulfides can also be produced by reduction of sulfates

(SO42-).

Over 2,000 minerals have been identified, but about 20 of them are common and fewer than 10 account

for over 90% of the earth’s crust by mass. The most common minerals are compounds such as the

silicates, carbonates, oxides and the sulfides. There are different types of sulfides which are categorized

in the – sulfide class. The sulfide class consists of the subclasses: simple sulfides, arsenides, antimonides,

tellurides selenides and sulfosalts.

Many sulfide minerals are economically important as metal ores. As mentioned above these sulfides will

be mined for both their sulfur content and their valuable metal content. Some common sulfides include:

Sulfide / Mineral Name Use

Silver sulfide / Argentite Major source of silver

Iron sulfide / Pyrite Major source of Iron

Copper iron sulfide / Major source of Copper

Chalcopyrite

Lead sulfide / Galena Major source of Lead

Mercury sulfide / Major source of Mercury

Cinnabar

Sulfate (SO42-)

Sulfate (SO42-) is composed of a central sulfur atom (S) surrounded by four

equivalent oxygen atoms (O) with two additional electrons in its valence

shell. This makes sulfate a fully oxidized polyatomic anion of sulfur with a

negative two (2- ) charge.

Natural Sources of Sulfates

Like sulfide, sulfate is found in nature bonded to other elements (usually a metal or hydrogen ion), and

are referred to as sulfates or sulfate minerals. Sulfates (SO42-) are salts of sulfuric acid (H2SO4) formed by

replacing one or both hydrogen ions with a metal or a radical (i.e. sodium sulfate - Na2SO4).

Sulfates commonly form in settings where highly saline waters slowly evaporate. Sulfates also occur

along side sulfides, forming by means of precipitation in hydrothermal fluids or other aqueous solutions.

And finally, sulfates can form due to the oxidation of sulfide minerals.

Sulfates are widely distributed in nature and are a part of naturally occurring minerals in some soil and

rock formations that contain groundwater. In these situations the sulfate minerals dissolve over time

(water soluble) and are released into groundwater. Therefore most drinking water supplies contain

traces of sulfates.

Sulfates, like sulfides, have their own category called the sulfate class. About 200 distinct kinds of

sulfate are recorded in mineralogical literature, but most of them are of rare and local occurrence. The

most common sulfates are used by manufacturers in a wide variety of products. Some of the more

common sulfates and their uses are:

Sulfate / Mineral Name Uses

Strontium sulfate / Colorant in pyrotechnics and

Celestine ceramics

Barium sulfate / Radiocontrast agent for

Barite X-ray images, used in paint

Hydrated calcium sulfate / Plaster of paris, cast, molds,

Gypsum wall plaster, wall board,

cement

Copper sulfate / Algaecide

Chalcanthite

Magnesium sulfate / Therapeutic bath salts

Epsomite

Sulfates are also use in a variety of personal care products such as - toothpaste, hair curling products

and shampoo.

Because both soil and groundwater contain sulfates it is only natural that sulfate will be found in

vegetation. This will be covered in more detail in the next section.

Man-Made Sources of Sulfate

Sulfates not only occur naturally in minerals, soil, groundwater, sea spray and food, but sulfates are also

a by-product of many industrial emissions and therefore can be found in the atmosphere. This occurs

indirectly from the combustion of sulfur containing fuels, by industry and individuals, which generates

emissions of sulfur dioxide. Once sulfur dioxide (SO2) enters the atmosphere, it goes through a series of

conversions and is subsequently oxidized or converted to sulfate (SO42-).

Mining is another source of atmospheric sulfates. Mining causes physical disturbances resulting in the

exposure of large quantities of mineral sulfides to atmospheric oxygen. This exposure causes the sulfides

(S2-) to again become oxidized in to sulfate (SO42-).

In the atmosphere sulfate, an anion with a (2- ) charge will attract other cations (+) elements to become

electrochemically stable. Sulfate may bind with ammonium (NH4+), calcium (Ca 2+) or hydrogen (H+).

Sulfate, in the atmosphere occurs as microscopic particles and can serve as condensation nuclei for the

formation of ice crystals, which may settle from the atmosphere as rain or snow. If the sulfate is bonded

to hydrogen the precipitation will be acidic and therefore contributing to the phenomena of acid rain.

Sulfur Compounds

Along with sulfate there are numerous chemical compounds containing sulfur. Listed and described

below are several that are relevant to this report.

Sulfur Dioxide (SO2)

Sulfur dioxide (SO2) is composed of one sulfur atom (S) and two oxygen

atoms (O), making it an oxide. Sulfur dioxide, at room temperature is a

toxic, non-flammable, colorless gas that is heavier than air. It has a

strong pungent, irritating odor familiar to the smell of a just struck

match.

Natural Sources of Sulfur Dioxide

Sulfur dioxide is released into the atmosphere from a variety of natural sources. Some of these natural

sources are:

Geothermal activity – hot springs and volcanic gases

Natural decay of vegetation on land, in

wetlands and oceans

Forest fires

Scientist estimate that natural sources release 80 to 290 million tons of sulfur dioxide into Earth’s

atmosphere annually. Volcanoes being the major natural source, for example: in 1991 Mt. Pinatubo in

the Philippines released between 15 and 30 million tons of sulfur dioxide into the atmosphere.

Man Made Sources of Sulfur Dioxide

Large emissions of sulfur dioxide are also associated with human activities. Sulfur dioxide is a primary air

pollutant (waste gas), released directly into the atmosphere resulting from the burning of sulfur and

sulfur compounds found in fossil fuels. Scientists estimate that humans contribute an additional 70 to

100 million tons of sulfur dioxide into the Earth’s atmosphere annually. The most common man-made

sources of sulfur dioxide emissions come from:

Power plants – burning of coal accounts for over 60% of human sulfur dioxide emissions

Commercial-Institutional heating plants

Industrial boilers

Petroleum refining

Metal smelters - smelting of sulfide ores

Pulp mills - production of paper products

Cement factories – heating of calcium sulfate

Textile plants - bleaching

Food processing plants - production of food

preservatives

Waste incineration plants

Homes – burning fuel oil to heat homes

Vehicle Exhaust (gasoline, diesel)

Sulfur dioxide (SO2) originates either directly from the

combustions of fossil fuels or from the oxidation of

hydrogen sulfide (H2S) which is also a by-product of some

industries. Sulfur dioxide is readily removed from the

atmosphere being oxidized fairly readily to sulfur trioxide

(SO3), then into sulfuric acid (H2SO4), and then to sulfate.

Uses of Sulfur Dioxide

Sulfur dioxide is both a pollutant by-product of some industries and a manufactured product of other

industries (chemical). In chemical manufacturing sulfur dioxide can be liquefied under moderate

pressures at room temperatures. Therefore, sulfur dioxide is handled and transported as a colorless

liquefied compressed gas which easily dissolves in water.

The chief use of manufactured sulfur dioxide (SO2) is as feedstock for the preparation and production of

further sulfur compounds - sulfur trioxide (SO3), sulfites (S32-) and especially the end product of sulfuric

acid (H2SO4). However, sulfur dioxide is also used as an industrial reducing agent, a solvent disinfectant,

a refrigerant, a bleaching agent, and a fumigant.

After listing the above usages it might surprise one to learn that sulfur dioxide is also used as a food

preservative. Foods are either exposed to the gaseous form or are dipped in a liquid form of sulfur

dioxide. Foods such as dried fruit, vegetables, meats, wine and beer may undergo this form of food

preservation. Interestingly, sulfur dioxide is also used in the manufacturing of sulfites which too are food

preservatives. Together sulfur dioxide and sulfites are heavily used throughout the food and restaurant

industry as a preservative agent in foods.

Problems with Sulfur Dioxide

Sulfur dioxide (SO2) is chemically unstable; therefore when it enters the atmosphere it will react with

oxygen (O) to produce sulfur trioxide (SO3). Sulfur trioxide, which is extremely hygroscopic (absorbs

moisture), may then react with atmospheric water (H2O) to produce a mist of sulfuric acid (H2SO4). At

this point sulfuric acid may either fall to the ground as precipitation (acidic) or it may further react with

metal oxides, as part of dust particles, to form sulfates (SO42-) such as ammonium sulfates (also acidic).

Along with nitrogen, sulfuric acid and sulfates are all components of acid rain. Acid rain lowers the pH

(more acidic) of soil and freshwater bodies, sometimes resulting in substantial damage to the

environment and in the chemical weathering of statues and structures.

Sulfur Trioxide (SO3)

Sulfur trioxide (SO3) is composed of one sulfur atom (S) and three

oxygen atoms (O) making it another oxide. Sulfur trioxide, at room

temperature, is a very reactive colorless gas. It acts as an oxidizing

agent.

Sulfur trioxide (SO3) can be put directly into the air as a primary

pollutant, but it more commonly builds up as a secondary air

pollutant following the oxidation reactions of sulfur dioxide (SO2). Once sulfur trioxide forms it rapidly

combines with water droplets (H2O) in the atmosphere to form sulfuric acid (H2SO4), which as

mentioned is a major component of acid rain.

Sulfur trioxide, like sulfur dioxide, is a by-product of industrial emissions as well as a manufactured

product. Sulfur trioxide is prepared commercially, as a colorless liquid, on a large scale by catalytic

oxidation of sulfur dioxide and converted to its end use of sulfuric acid.

Sulfurous Acid (H2SO3)

Sulfur forms about 16 oxygen-bearing acids. Of these acids,

sulfurous acid (H2SO3) and sulfuric acid (H2SO4) in particular

are of considerable importance to the chemical industry.

Sulfurous acid (H2SO3) is composed of a central sulfur atom (S)

surrounded by three equivalent oxygen atoms (O) with two of

the oxygen atoms bonded to a hydrogen atom (H).

Sulfurous acid (H2SO3) is formed when sulfur dioxide (SO2) is dissolved in water (H2O). The compound is

only known in aqueous solution or in the form of its salts and is not isolated in its pure state. The

dissociation or breaking up of sulfurous acid results in salts called sulfites (SO32-).

In chemical manufacturing sulfurous acid is produced as a clear colorless liquid with a pungent odor

similar to sulfur dioxide. Sulfurous acid is a weak acid and is used primarily as a chemical-reducing and

bleaching agent. It is also used as a disinfectant/antiseptic and has been used in skin lotions and nasal

and throat sprays.

Sulfuric acid (H2SO4)

Sulfuric acid (H2SO4) is composed of a central sulfur atom (S)

surrounded by four equivalent oxygen atoms (O) with two of the

oxygen atoms bonded to a hydrogen atom (H). Sulfuric acid is a

colorless, odorless, extremely corrosive, oily liquid.

By now you know that sulfuric acid is formed by way of sulfur

dioxide (SO2) entering the atmosphere and becoming oxidized

to sulfur trioxide (SO3). Sulfur trioxide then reacts with

atmospheric moisture (H2O) and forms sulfuric acid (H2SO4). And as you also know, sulfuric acid is a

major component of acid rain and its many destructive features.

It should also be pointed out that acid rain may occur as any type of precipitation – rain, freezing rain,

snow, sleet and hail. What is noteworthy is if the process of forming sulfuric acid takes place close to the

surface acid fog will form. Acid fog is capable of causing the same extreme environmental harm as acid

rain but more importantly it can be particularly dangerous to people because it is easily inhaled.

Use of Sulfuric Acid

While environmentalist, politicians and industry leaders battle over how to reduce the amount of

sulfuric acid that is formed in the atmosphere, the chemical industry, on the other hand is continually

increasing its production of sulfuric acid. This is because sulfuric acid is one of the most important

industrial raw materials produced by the chemical industry. Sulfuric acid production is the major end

product of sulfur and more sulfuric acid is produced in the United States every year than any other

chemical - nearly 40 million tons are used each year.

Sulfuric acid is used and enters into the manufacture of a wide range of substances, although it rarely

forms any part of the product that is sold to the consuming public. The principal use for sulfuric acid is in

the extraction of phosphate ores for the production of phosphate fertilizers as well as trisodium

phosphate for detergents.

Other applications of sulfuric acid include its use in:

Oil refining Pigments and dyes

Waste water processing Paint

Metal/mineral extraction and purification Explosives

Lead-acid batteries Pharmaceuticals

Sulfite (SO32-)

Sulfite (SO32-) is composed of one sulfur atom (S) bonded to

three oxygen atoms (O) with a negative two (2- ) charge.

This makes it another polyatomic anion of sulfur. Sulfites are the

salts of sulfurous acid (H2SO3), formed by replacing

one or both of the hydrogen atoms with a metal or a radical

(i.e. sodium sulfite - Na2SO3). In general, sulfites are moderately

strong reducing agents.

Sulfites are naturally occurring compounds that nature uses to prevent microbial growth. They are

found naturally on grapes, onions, garlic and many other plants.

Though sulfites can be found in nature, they too are manufactured on a large scale by the chemical

industry. Sulfur dioxide gas (SO2) dissolves fairly readily in water (H2O) to create a solution of sulfurous

acid (H2SO3). Sulfurous acid gives rise to two series of salts – a normal sulfite ion (SO32-) and a hydrogen

sulfite ion (HSO3-).

Sulfites are heavily used by the food industry as a preservative. Sulfites are commonly found in wines (to

prevent spoilage and oxidation) dried fruits, dried potato products, jams, canned vegetables, soup mixes,

condiments, etc. etc. etc.. Sulfites are used in pharmaceuticals to maintain the stability and potency of

certain medications. And sulfites are used as a reducing agent for bleaching and to prevent rust.

There are several types of sulfites the most important of which is

sodium sulfite (Na2SO3). Sodium sulfite produced as a white crystalline

or powder is readily soluble in water. It is a strong reducing agent and

reacts with oxidants. It is primarily used in pulp

and paper industry. Though it also has uses in

water treatment as an oxygen scavenger agent,

in the photographic industry to protect

developer solutions from oxidation and in textile

industry as a bleaching agent. Sodium sulfite is

also extensively used for preserving foods as well.

Hydrogen Sulfide (H2S)

Hydrogen Sulfide (H2S) is composed of one sulfur atom (S) and two

hydrogen atoms (H). Hydrogen sulfide is a colorless, flammable,

extremely hazardous gas (more than sulfur dioxide) with a rotten egg

smell. Hydrogen sulfide is heavier than water and may travel along the

ground. Hydrogen sulfide is a very reactive reducing agent and a weak

acid.

Sulfur itself has essentially no scent. As for the rotten egg odor that is detected, it occurs when water

(H2O) mixes with sulfur (S) producing hydrogen sulfide gas (H2S). Although only small amounts of

hydrogen sulfide form this way, it is a powerful odor producer.

“Danger Will Robinson” - although the smell of hydrogen sulfide is very pungent at first, it quickly

deadens the sense of smell, so potential victims may be unaware of its presence until symptoms begin

or death occurs.

Natural Sources of Hydrogen Sulfide Hydrogen Sulfide occurs frequently in nature and can be found occurring naturally in:

Crude petroleum

Natural gas - which can contain up to 28% hydrogen sulfide (this is why natural gas wells are

referred to as “sour gas” wells - offensive stench).

Volcanic and hot spring vapors.

Oceanic surface waters, where most of the marine metabolic turnover takes place.

Swamps, shallow lakes, ground water, deep wells, plumbing systems, water softeners and water

heaters. Hydrogen sulfide is produced in these areas by sulfur-reducing bacteria (uses sulfur as

an energy source) which break-down sulfur containing organic materials. The other common

factor with these areas is they are deficient in oxygen.

Mammals – flatulence and waste.

Man-Made Sources of Hydrogen Sulfide

Hydrogen Sulfide, like the other sulfur compounds, is both a pollutant emitted into the air by industry

and a valuable raw material to be put to further use. Industries that are known to release the pollutant

hydrogen sulfide (usually along with sulfur dioxide) into the atmosphere are:

Petroleum and natural gas drilling and refining

Wastewater treatment plants

Paper/pulp mills.

Other sources of industrial hydrogen sulfide are Coke ovens (heating of coal) and Tanneries (leather).

Whether the source of hydrogen sulfide originates from man or nature, once in the atmosphere, it will

through a series of conversions slowly be oxidized to sulfate.

Large amounts of hydrogen sulfide are recovered, as a raw material, in the removal or separation of

sulfur from petroleum and natural gas during production. A substantial amount of this hydrogen sulfide

is converted to solid elemental sulfur. The remaining hydrogen sulfide is used extensively in chemical

laboratories as an analytical reagent (a pure chemical) usually as a liquid compressed gas.

The Sulfur Cycle – Putting it all Together

Are you now confused? Is your head spinning? If so, it’s because the interaction between sulfur and the

sulfur compounds within the environment takes place as a cycle. A big cycle where no one component

works independently, they’re all connected in one form or another.

As an essential element, sulfur is constantly being converted, transformed and transported throughout

the environment. The movements follow certain well defined pathways which collectively link up into

one system in what scientist refer to as - the sulfur cycle. Below on the next page is a simplified

conceptual model of the sulfur cycle.

The next step is to see how this environmental sulfur cycle interacts with the human body’s own

naturally occurring, bio-chemical sulfur cycle. This will be the subject of the next report. That’s when

your head is really going to spin.

Image (below): Sulfur Cycle