Sulfur Cycle

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Sulfur Cycle

• The sulfur cycle is relatively complex in that it
  involves several gaseous species, poorly soluble
  minerals, and several species in solution.
• The sulfur cycle has both gaseous and sedimentary
  phases. Sulfur occurs in a gaseous form, as H2S
  and SO2 and a solid form as sulfate, sulfide and
  organic sulfur in soil and in the body of living
  organisms.
• The residence time of sulfur in the atmosphere is
  very small and its main reserve pool is found in
  the soil. Sulfur enters the atmosphere from
  several sources
• - the combustion of fossil fuels
• - volcanic eruption
• - exchange at the surface of the oceans
• - gases released by decomposition

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• The sulfur enters the atmosphere initially as
  H2S, which quickly interact with O2 to form SO2.
  The SO2 and H2S in atmosphere return to the
  surface in rainwater/soil as sulfate or sulfuric
  acid along with rain.
• The sulfur cycle is tied with the oxygen cycle in
  that sulfur combines with oxygen to form gaseous
  SO2, an atmospheric pollutant, and soluble SO42-.
• Among the significant species involved in the
  sulfur cycle are gaseous H2S volatile dimethyl
  sulfide (CH3)2S, released to the atmosphere by
  biological processes in the ocean mineral
  sulfides, such as PbS H2SO4, the main
  constituent of acid rain and biologically bound
  sulfur in sulfur-containing proteins.
• The most significant part of the sulfur cycle is
  the present of pollutant SO2 gas and H2SO4 in the
  atmosphere. SO2 is a toxic gaseous air pollutant
  from the combustion of sulfur-containing fossil
  fuels.

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• Sulfur in soluble form is taken up by plants and
  incorporated through a series of metabolic
  processes, starting with photosynthesis, into
  sulfur-bearing amino acids.
• Sulfur found in living organisms is essential for
  the synthesis of certain amino acids such as
  cystine, cyctein, and methionine, the peptide
  glutathione, enzyme cofactors like thiamine,
  biotine, thioctic acid and certain vitamins.
• From the producers, sulfur in amino acid is
  transferred to consumers through food chains.
  With the decay of dead bodies of organisms and
  plants, sulfur comes back to the soil, ponds,
  lakes and seas, where bacteria release it as
  hydrogen sulfide or sulfate.
• In the decomposer system, fungi like Aspergillus
  and Neurospora under aerobic conditions, and
  bacteria like Escherichia and Proteus in
  anaerobic conditions, are responsible for the
  decomposition of sulfur containing proteins.

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• In anaerobic conditions and sediments, H2S is
  formed by sulphate reducing bacteria like
  Desulphonovibrio desulfuriccans. The bacteria
  utilize the oxygen in the sulfate molecule to
  obtain energy and in turn reduce the sulfate in
  deep sediments to H2S gas.
• SO42- 2CH2O 2H H2S 2CO2
  2H2O
• Under anaerobic condition, sulfur will
  precipitate as ferrous sulfide (FeS2) in the
  iron-rich materials. The FeS2 is highly insoluble
  and it is firmly held in mud and wet soil.
• Some chemoautotrophic bacteria (Thiothrix and
  Thiobacillus chemoautotrophic), oxidize H2S to
  S and then S to sulfate.
• 2H2S O2 2H2O 2S energy
• 2S 3O2 2H2O 4 H 2SO42-
  energy

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• Some of the photosynthetic bacteria like the
  green sulfur bacteria (Chlorobium sp.) and purple
  sulfur bacteria (Chromatium sp.) utilize H2S as a
  source of hydrogen to reduce CO2 to produce
  glucose.
• CO2 H2S
  (CH2O)n S H2O Energy
• Some other nutrients, such as Fe, Cu, Ca, Co, Zn
  etc become available when react with S. For
  example, Fe is precipitated out as FeS2 and
  become available to organisms.

Solar energy, bacteria chlorophyll
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The Sulfur Cycle
Atmospheric sulfur, SO2, H2S, H2SO4, CS2, (CH3)2S
Interchange of atmospheric sulfur species with
other environmental spheres
Inorganic SO42- in both soluble and insoluble
forms
S oxidation
Elemental sulfur, S
H2S oxidation
Assimilation by organisms
Decomposition
Sulfides as H2S and as metal sulfides, such as FeS
Biological sulfur, including SH groups
Biodegradation
Microbial metabolism
S
Microbially produced organic sulfur in small
molecules, largely as SH and R-S-R groups
Xenobiotic sulfur like P- groups in insecticides
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The sulfur cycle. Note the 2 components
sedimentary and gaseous. Major sources from human
activity are the burning of fossil fuels and
acidic drainage from coal mines
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Acid Rain

• The term acid rain also known as acid
  precipitation is commonly used to mean the
  deposition of acidic components in rain, snow,
  dew, or dry particles. The more accurate term is
  "acid precipitation." 1 Acid rain occurs when
  sulfur dioxide and nitrogen oxides are emitted
  into the atmosphere, undergo chemical
  transformations and are absorbed by water
  droplets in clouds. The droplets then fall to
  earth as rain, snow, mist, dry dust, hail, or
  sleet. This increases the acidity of the soil,
  and affects the chemical balance of lakes and
  streams

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Next

• Acid rain is defined as any type of precipitation
  with a pH that is unusually low.4 Dissolved
  carbon dioxide dissociates to form weak carbonic
  acid giving a pH of approximately 5.6 at typical
  atmospheric concentrations of CO2.5 Therefore a
  pH of less than 5.6 has sometimes been used as a
  definition of acid rain.6 However, natural
  sources of acidity mean that in remote areas,
  rain has a pH which is between 4.5 and 5.6 with
  an average value of 5.0 and so rain with a pH of
  less than 5 is a more appropriate definition.7
  The US EPA says, "Acid rain is a serious
  environmental problem that affects large parts of
  the US and Canada" 8 Acid rain accelerates
  weathering in carbonate rocks and accelerates
  building weathering. It also contributes to
  acidification of rivers, streams, and forest
  damage at high elevations. When the acid builds
  up in rivers and streams it can kill fish

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Emissions of chemicals leading to acidification

• The most important gas which leads to
  acidification is sulfur dioxide. Emissions of
  nitrogen oxides which are oxidized to form nitric
  acid are of increasing importance due to stricter
  controls on emissions of sulfur containing
  compounds. 70 Tg(S) per year in the form of SO2
  comes from fossil fuel combustion and industry,
  2.8 Tg(S) from wildfires and 7-8 Tg(S) per year
  from volcanoes
• The principal cause of acid rain is sulfuric and
  nitrogen compounds from human sources, such as
  electricity generation, factories and motor
  vehicles. Coal power plants are one of the most
  polluting. The gases can be carried hundreds of
  kilometres in the atmosphere before they are
  converted to acids and deposited. Factories used
  to have short funnels to let out smoke, but this
  caused many problems, so now, factories have
  longer smoke funnels. The problem with this, is
  those pollutants get carried far off, where it
  creates more destruction.

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Chemistry in cloud droplets

• When clouds are present the loss rate of SO2 is
  faster than can be explained by gas phase
  chemistry alone. This is due to reactions in the
  liquid water droplets
• Hydrolysis
• Sulfur dioxide dissolves in water and then, like
  carbon dioxide, hydrolyses in a series of
  equilibrium reactions
• SO2 (g) H2O ? SO2H2O
• SO2H2O ? HHSO3-
• HSO3- ? HSO32-
• Oxidation
• There are a large number of aqueous reactions
  that oxidise sulfur from S(IV) to S(VI), leading
  to the formation of sulfuric acid. The most
  important oxidation reactions are with ozone,
  hydrogen peroxide and oxygen (reactions with
  oxygen are catalysed by iron and manganese in the
  cloud droplets).

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Acid deposition
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Surface waters and aquatic animals

• Both the lower pH and higher aluminium
  concentrations in surface water that occur as a
  result of acid rain can cause damage to fish and
  other aquatic animals. At pHs lower than 5 most
  fish eggs will not hatch and lower pHs can kill
  adult fish. As lakes become more acidic
  biodiversity is reduced. Acid rain has eliminated
  insect life and some fish species, including the
  brook trout in some Appalachian streams and
  creeks

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Effect

• Soils
• Soil biology can be seriously damaged by acid
  rain. Some tropical microbes can quickly consume
  acids13 but other microbes are unable to
  tolerate low pHs and are killed. The enzymes of
  these microbes are denatured (changed in shape so
  they no longer function) by the acid. The
  hydronium ions of acid rain also mobilize toxins
  and leach away essential nutrients and minerals.
• Forests and other vegetation
• Acid rain can slow the growth of forests, cause
  leaves and needles to turn brown and fall off and
  die. In extreme cases trees or whole areas of
  forest can die. The death of trees is not usually
  a direct result of acid rain, often it weakens
  trees and makes them more susceptible to other
  threats. Damage to soils (see above) can also
  cause problems. High altitude forests are
  especially vulnerable as they are often
  surrounded by clouds and fog which are more
  acidic than rain.
• Human health
• Some scientists have suggested direct links to
  human health, but none have been proven.2.
  However, fine particles, a large fraction of
  which are formed from the same gases as acid rain
  (sulfur dioxide and nitrogen dioxide), have been
  shown to cause illness and premature deaths such
  as cancer and other deadly diseases15 For more
  information on the health effects of aerosol.

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• Oxygen Cycle
• Oxygen is present in large quantities (29.95) in
  the atmosphere There are 2 significant sources of
  atmospheric oxygen
• Breakup of water vapor through a process driven
  by sunlight. H2O is dissociated to produce H2 and
  O2. Most of the H2 escapes into space.
• (ii) Photosynthesis process. O2 is produced by
  green plants and consumed by both plants and
  heterotrophic organisms in respiration, so that
  there is a balance between the amount of O2
  production and utilization.

• The oxygen cycle involves the interchange of
  oxygen between the elemental form of gaseous O2
  and chemically bound O in CO2 and H2O, minerals,
  and organic matter. It is strongly tied with
  other elemental cycles, particularly the carbon
  cycle.

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• Elemental oxygen becomes chemically bound by
  various energy-yielding processes, particularly
  combustion and metabolic processes in organisms.
• Combustion of fossil fuels such as CH4
• CH4 O2 CO2 2H2O
• Elemental oxygen also oxidizes inorganic
  substances such as Fe(II) in minerals
• 4FeO O2 2Fe2O3
• Dissolved oxygen in water is the source of oxygen
  for aquatic life.
• The other main reservoirs of O2 are H2O and CO2.
  All the reservoirs are linked through
  photosynthesis. O2 is also biologically
  exchangeable in sulfates and nitrates, which
  organisms transform to ammonia and hydrogen
  sulfide.

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• Some carbon dioxide combines with calcium to form
  carbonates. Oxygen combines with nitrogen
  compounds to form nitrates, with iron to form
  ferric oxides, and with other minerals to form
  oxides. In these states, oxygen is temporarily
  withdrawn from circulation.
• Another phase of oxygen is ozone layer of the
  outer stratosphere of the atmosphere which
  protects life from ionizing short wave radiations
  (ultraviolet).

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The Oxygen Cycle
Atmospheric oxygen O2
Organic molecules C6H12O6
Components of living mater
Respiration
Photosynthesis
Nutrient uptake
Nutrient pool
CO2
H2O
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Oxygen exchange among the atmosphere, geosphere,
hydrosphere and biosphere