Transcript SulfurOxygen_Cycle

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
•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.
•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.
•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- + 2{CH2O} + 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
2S + 3O2 + 2H2O
2H2O + 2S + energy
4 H+ + 2SO42- + energy
•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
Solar energy,
bacteria chlorophyll
(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.
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
Biological sulfur,
including –SH groups
Sulfides as H2S and as
metal sulfides, such as FeS
Biodegradation
Microbial metabolism
Microbially produced organic
sulfur in small molecules,
largely as –SH and R-S-R
groups
S
Xenobiotic sulfur like –Pgroups in insecticides
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
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
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
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.
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 ⇌ SO2·H2O
• SO2·H2O ⇌ H++HSO3• HSO3- ⇌ H++SO32• 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).
Acid deposition
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
Effect
• Soils
• Soil biology can be seriously damaged by acid rain. Some tropical microbes
can quickly consume acids[13] 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 diseases[15] For more information on the health effects of
aerosol.
Oxygen Cycle
• Oxygen is present in large quantities (29.95%) in the atmosphere
There are 2 significant sources of atmospheric oxygen:
(i) 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.
•Elemental oxygen becomes chemically bound by various energyyielding 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.
•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).
The Oxygen Cycle:
Atmospheric oxygen
O2
Organic molecules
C6H12O6
Components of
living mater
Respiration
Photosynthesis
Nutrient uptake
Nutrient pool
CO2
H2O
Oxygen exchange among the atmosphere, geosphere, hydrosphere
and biosphere