The sulfur cycle describes the movement of sulfur through the biosphere and lithosphere. Key points include: - Sulfur is released by volcanoes as hydrogen sulfide and sulfur dioxide gases. - Sulfur is oxidized to sulfate and reduced to sulfide as it moves between organisms and the environment. - Human activities like burning fossil fuels release sulfur dioxide into the atmosphere.

1. Sulfur Cycle
• Most sulfur is buried in rocks and minerals
underground
• Hydrogen sulfide (H2S) and sulfur dioxide (SO2)
are released by volcanoes.
• The most reduced forms of sulfur are the
sulfides, such as the odourus gas hydrogen
sulphide (H2S).
• Like the ammonium ion of the nitrogen cycle,
this is a reduced compound that generally forms
under anaerobic conditions.

2. Slide 42
Slide 42
Slide 42
Figure 4-34
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Sulfur
Hydrogen
sulfide
Sulfate salts
Plants
Acidic fog and
precipitation
Ammonium
sulfate
Animals
Decaying
matter
Metallic
sulfide
deposits
Ocean
Dimethyl
sulfide
Sulfur dioxide Hydrogen
sulfide
Sulfur trioxide Sulfuric acid
Water
Ammonia
Oxygen
Volcano
Industries
SULFUR CYCLE

3. • Steps of the sulfur cycle are:
• Mineralization of organic sulfur into inorganic forms, such as
hydrogen sulfide (H2S), elemental sulfur, as well as sulfide minerals.
• Oxidation of hydrogen sulfide, sulfide, and elemental sulfur (S) to
sulfate (SO4(2-)).
• Reduction of sulfate to sulfide.
• Incorporation of sulfide into organic compounds (including metal-
containing derivatives).

4. Sulfur Cycle
SO4
-2
Sulfate Reduction
(Assimilitory)
Organic Sulfur
H2S
Mineralization
Sulfur Oxidation
Sulfate Reduction
(Dissimilitory)
Elemental Sulfur
Sulfur Oxidation
Sulfur Reduction

5. SO4
-2
ATP
APS
2 ADP
2 ATP
SO3
-2
S3O6
S2O3
-2
Sulfate Reduction

6. Sulfur Reduction
Thiosulfate Disproportionation
S2O3
-2 + H2O SO4
-2 + HS- + H+
S0 + H2 HS- + H+

7. HUMAN IMPACTS TO SULFUR CYCLE
Approximately 1/3 of all sulfur emitted into atmosphere comes from
human activities.
• Burning sulfur containing coal and oil to produce electric power (SOx
= acid deposition).
• Convert sulfur-containing metallic ores into free metals such as
copper, lead, and zinc releasing sulfur dioxide into the environment.
• Refining petroleum – (SOx emissions)
• Smelting to convert sulfur compounds of metallic minerals into free
metals (Cu, Pb, Zn)
• Industrial processing.

8. IMPORTANCE OF SULFUR CYCLE
1. Sulfur is a component of most proteins and some vitamins.
2. Sulfate ions (SO4
2- ) dissolved in water are common in
plant tissue. They are part of sulfur-containing amino
acids that are the building blocks for proteins.
3. Sulfur bonds give the three dimensional structure of amino
acids.
4. Many animals, including humans, depend on plants for
sulfur-containing amino acids.

9. Sulfur Cycle
• Sulfur cycle is the natural cycle which includes the
mineralization of organic sulfur to sulfide, oxidation of this
to sulfate, and reduction of this to sulfide followed by
microbial incorporation of this into organic compounds.

10. Sulfur Cycle
• The sulfur cycle includes both gases and solids.
• Hydrogen sulfide gas (H2S) is released into the atmosphere
by volcanic eruptions, hot springs , and the anaerobic decay
of sulfur-containing biological material in swamps, bogs, and
tidal flats.
• Certain marine algae produce dimethyl sulfide, (CH3)2S, a
volatile compound that enters the atmosphere as tiny
droplets.
• Sulfur dioxide gas is released into the atmosphere by
volcanoes and also forms when dimethyl sulfide reacts with
oxygen gas.
• The burning of fossil fuels, such as coal and oil releases sulfur
dioxide into the atmosphere.

11. Sulfur Cycle
• Sulfur dioxide reacts with atmospheric oxygen to produce
sulfur trioxide (SO3).
• Some sulfur trioxide reacts with tiny water droplets to
form sulfuric acid (H2SO4).
• Sulfur oxides also react with ammonia to produce tiny
particles of ammonium salts.
• The winds carry droplets of sulfuric acid and particles of
sulfate salts, which then fall to the earth in acid
deposition.
• By adding sulfur compounds to the air, the use of fossil
fuels increases the rate of acid deposition.
• All living things require sulfur to make proteins.

12. Sulfur Cycle
• Plants get sulfur by taking up ions of sulfate salts (SO4
2-)
from the soil.
• Animals get sulfur by eating plants, and all living things
release sulfur compounds when they decay.
• Decomposition releases sulfate salts (SO4
2-), which can be
taken up by plants, as well as gaseous hydrogen sulfide.

13. Sulfur Cycle
• Some hydrogen sulfide enters the atmosphere.
• But when decay occurs in an oxygen-free environment,
anaerobic bacteria break down hydrogen sulfide and
release sulfur gas (S2).
• Oxygen-requiring bacteria can incorporate sulfur into
sulfate salts, which can be taken up by plants and enter
the food chain once again.