Sulpher cycle

Sulpher cycle

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  HAFIZ MUHAMMAD WASEEM LAHORE PAKISTAN
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  Applied Ecology ZOOL3118 UNIVESITY OFEDUCATION LAHORE PAKISTAN
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  Key Topic • BiogeochemicalCycles • Sulpher cycle “Bio” refers to living organisms and “geo” to the rocks, soil, air and water of the earth
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  • According tothe law of conservation of matter, matter is neither created nor destroyed • The earth neither receives matter nor does it lose to outer space • Elements must have been used over and over • The atoms of each element is taken from the environment, made a part of an organism and returned to the environment to be used over again • The cyclic movements of chemical elements of the biosphere between the organism and the environment are referred to as biogeochemical cycles Biogeochemical cycles
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  Phases of BiogeochemicalCycles • Organic Phase: Movement of elements through biotic communities via food chains • Abiotic Phase: Movement of elements external to food chain • Flow in the abiotic phases is much slower than in the organic phase
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  Phases of BiogeochemicalCycles • There are two classes of abiotic phases in biogeochemical cycles • Sedimentary phase • Atmospheric phase • In some cycles, such as nitrogen, the atmospheric phase is more important than the sedimentary------- atmosphere reservoir cycles • In others, such as phosphorus, the atmospheric phase is essentially non- existent------sediment-reservoir cycles • In still others, such as sulphur, both phases are present
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  TYPES OF BIOGEOCHEMICALCYCLES • There are two types of biogeochemical cycles, • Gaseous biogeochemical cycles (Atmosphere and ocean) • Sedimentary biogeochemical cycles (Earth crust) • Both involve biotic and abiotic agents • Both are driven by the flow of energy • Both are tied to the water cycle
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  Sulphur cycle • Sulphuris an essential part of protein and amino acids and is characteristic of organic compounds • It exists in a number of states e.g., • elemental sulphur, S, • sulphides, • sulphur monoxide, • Sulphite • sulphates • Of these three are important in nature : elemental sulphur, sulphides and sulphates
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  Sulphur cycle • Thesulphur cycle is both sedimentary and gaseous • The sedimentary phase of sulphur cycle is long-termed • sulphur is tied up in organic and inorganic deposits • From these deposits, it is released by weathering and decomposition, and is carried to terrestrial and aquatic ecosystems in a salt solution
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  • Atmospheric (gaseous)phase of sulphur-cycle is less pronounced • Sulphur enters the atmosphere from several sources— • the combustion of fossil fuels, • volcanic eruption, • the surface of the oceans and gases released by decomposition. • Initially sulphur enters the atmosphere as hydrogen sulphide, H2S, which quickly oxidizes into another volatile form, sulphur dioxide, SO2. • Atmospheric sulphur dioxide, soluble in water, is carried back to earth in rainwater as weak sulphuric acid, H2SO4.
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  • Whatever itssource, sulphur in a soluble form, mostly as sulphate (SO4 --) is absorbed through plant roots • where it is incorporated into certain organic molecules, such as some amino acids (e.g., cystine) and proteins • From the producers the sulphur in amino acids is transferred to the consumer animals, with excess being excreted in the faeces
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  • Excretion anddeath carry sulphur in living material back to the soil and to the bottoms of water bodies where the organic material is acted upon by bacteria of detritus food chain. • The sulphydryl group (–SH) of amino acids (e.g., L-cysteine) is separated from the rest of the molecule as hydrogen sulphide (H2S) by most decomposing bacteria • In an aerobic environment, the hydrogen sulphide is oxidized to sulphate by bacteria specially adapted to perform this conversion : H2S + 2O2 → SO4 -- + 2H+
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  • The sulphateproduced then can be reused by the autotrophs • In anaerobic environments, such as bottom of certain lakes, it is impossible to oxidize sulpide • But infrared radiation is present in these environments and photosynthetic bacteria that use it to manufacture carbohydrates and oxidize sulphide either to elemental sulphur or to sulphate: 6CO2 + 12H2S + hv → C6H12O6 + 6H2O + 12S 6CO2 + 12H2O + 3H2S + hv → C6H12O6 +6H2O+3SO4 --+6H+
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  • Elemental sulphurcan also be utilized by other bacteria to form sulphate. If oxygen is present, the reaction is quite rapid. 2S + 3O2 + 2H2O → 2SO4 = + 4H+ • Under anaerobic conditions, elemental sulphur can still be oxidized to sulphate by certain bacteria if nitrate is present • 6NO3 – + 5S + 2CaCO3 → 3SO4 = + 2 CaSO4 + 2CO2 + 3N2 • None of these bacterial reactions is unidirectional; under certain conditions, sulphate can also be reduced either to sulphide or to elemental sulphur by bacteria
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  • The sulphuris removed from the organic phase in the form of elemental sulphur which is insoluble and accumulates in sediments. • If iron is present in the sediment, it can combine with sulphide to form iron sulphides, all of which are highly insoluble : Fe++ + S= → FeS Fe(ionic) + 2S(ionic) → FeS2 (Ferrous sulphide or pyrite) • FeS2 is highly insoluble under neutral and alkaline conditions and is firmly held in mud and wet soil. • Some ferrous sulphide is contained in sedimentary rocks overlying coal deposits.
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  • Exposed tothe air in deep and surface mining, the ferrous sulphide oxidizes and in the presence of water produces ferrous sulphate and sulphuric acid : 2FeS2 + 7O2 + 2H2O → 2FeSO4 + 2H2SO4 12FeSO4 + 3O2 + 6H2O → 4Fe2(SO4)3 + 4Fe(OH3) • In this manner sulphur in pyrite rocks, suddenly exposed to weathering by man, discharges heavy slugs of sulphur, sulphuric acid, ferric sulphate and ferrous hydroxide into aquatic ecosystems. These compounds destroy aquatic life and cause acidic water
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  • Human activitieshave affected the sulfur cycle primarily by releasing large amounts of sulfur dioxide (SO2) into the atmosphere by: • Burning sulfur-containing coal and oil to produce electric power. • Refining sulfur containing petroleum to make gasoline, heating oil, and other useful products. • Conversion of sulfur containing metallic mineral ores into free metals such as copper, lead, and zinc. • Once in the atmosphere, SO2 is converted to droplets of sulfuric acid (H2SO4) and particles of sulfate (SO4 2) salts, which return to the earth as acid deposition