The Sulfur Cycle

The Sulfur Cycle
The sulfur cycle is the collection of processes by which
sulfur moves to and from minerals (including the waterways)
and living systems.
 In nature: it can be found as the pure element,
and as sulfide and sulfate minerals.
Commercial uses: fertilizers, gunpowder,
matches, insecticides, fungicides, vitamins,
proteins and hormones.
It is critical in the environment, climate and the
health of ecosystems.
Its the 10th most abundant element in the
universe and 7th most abundant element in our
Amino acids: Cystein and methionine
Biological importance
 Amino acids: Methionine and Cysteine
 Therefore important part of proteins, enzymes etc.
 Vitamins
WHERE is sulfur found?
 The majority of Earth's sulfur is stored:
 In rocks underground!
 In sulfur salts at the bottom of the ocean!
Sulfur Cycle
 In ground: most found in rocks, or salt in earth, or as
sediment at bottom of ocean
 Found as S, H2S, SO4-2, (NH4)2SO4
 Enter ground: Plants absorb, or left by acid deposition (fog or
 As SO4-2, (NH4)2SO4, and then turn H2S by bacteria, decay, and
plant use
 Stored: Ground, rock, ocean, somewhat in air
Sulfur Cycle
 Sulfur is transferred into biosphere then back into ground, or
from ground to atmosphere
 Microorganisms turn it into H2S (gas)
 Oxidized in atmosphere to SO2, and then to H2SO4 (an acid)
with water contact
 Mined ores released to atmosphere in factories as H2S and SO2
 Volcanoes and hot springs
Sulfur Cycle
 Deposited next in water
 Through precipitation, dry deposition, leaching
 Rainfall= deposited 73E12 grams sulfur in 1960
 SO4-2 leaches from soil into ocean as sediment
 H2SO4 falls into ocean
 Dimethyl Sulfide, carbonyl sulfide (biogenic gases), released by
plankton returns back into atmosphere (turns into SO2)
 Either re-evaporated, left as sediment for long time, or
deposited on land
 20E12 grams of sulfur a year deposited on land by sea
 When back on land, cycle repeats
The Atmospheric Portion
 Volcanic eruptions, breakdown of organic matter
in swamps and tidal flats, and the evaporation of
water, especially seawater, release sulfur directly
into the atmosphere.
 Sulfur eventually settles to earth or comes down
with rainfall.
Driving Force
 Driven by:
 constant addition of sulfur to environment by earths interior
 Human disturbance, addition of sulfur to atmosphere, (also dug
up from environment)
 Natural processes (incl. Biological, hydrological, due to sun
 Plant uptake, microbes (Desulfovibrio sp. or Desulfotomaculum
 Physical Weathering release of sulfides (HS-) or sulfates (SO4-3)
from minerals
 Biological transformations:
 Aerobic
sulfur-oxidizing bacteria ; sulfides are converted to sulfate (SO4-2)
sulfate is assimilated by plants and microbes
 Anaerobic
sulfate-reducing bacteria; sulfate converted to sulfides
 Aerobic or anaerobic
 Mineralization of organic S, release as either HS- or SO4-2
Human Activities
 The burning of fossil fuels and processing of
metals releases huge quantities of sulfur into
the atmosphere.
 Human activities are responsible for one-third
of all sulfur emissions and 90% of all sulfur
dioxide emissions.
 Sulfur dioxide emissions lead to acid rain as
sulfur dioxide reacts with water to form H2SO4
and sulfur trioxide reacts with water to form
Human Effect
 When mine ores, sulfur/sulfides released into soil
 Combustion of fossil fuels
 Release of SO2, causes acid rain, increases amount already
 28% of sulfur in rivers from pollution, mining, erosion, etc.
 Help move cycle but also upset balance- too much S means acid
 Hydrodesulphurization (refine hydrocarbons)- surplus of S
 Sulfur Cycle is important to biological and natural processes
although human’s role impacts nature in a negative way

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