Sulfur Cycle

1. 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

2. 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.

3. 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.

4. 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 2H2O + 2S + energy • 2S + 3O2 + 2H2O 4 H+ + 2SO42- + energy

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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.

11. 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).

12. Acid deposition

13. 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

14. 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.

15. 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.

16. 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.

17. 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).

18. The Oxygen Cycle: Atmospheric oxygen O2 Organic molecules C6H12O6 Components of living mater Respiration Photosynthesis Nutrient uptake Nutrient pool CO2 H2O

19. Oxygen exchange among the atmosphere, geosphere, hydrosphere and biosphere