14th element in terms of terrestrial abundance. Concentrated in the mantle and crust. Presence in the metallic core - hypothetical

Transcription

1 The sulfur cycle

2 14th element in terms of terrestrial abundance. Concentrated in the mantle and crust. Presence in the metallic core - hypothetical 9 isotopes: 32 S: stable 95.02% 33 S: stable 0.75% 34 S: stable 4.21% 36 S: stable 0.02% 35 S: t1/2 = 88 days S Standard: Canyon Diablo Troilite 34 S ( ) = ( 34 S/ 32 S) sample -1/ ( 34 S/ 32 S) CDT x 1000

3 Oxydation states sulfides transitional sulfates Dissolved forms H 2 S, HS - S SO 2- SO 2-4, HSO - 8 S 2 O Minerals FeS, MeS FeS 2 CaSO 4 Gases COS, CS 2 SO 2 SO 3

4 REDOX reactions in nature There are mixtures of components at different oxidation states and not separate cells. Different affinities to electrons of the dissolved species drive the reactions. The insertion of an inert electrode (Pt) into an aquatic solution will enable us to measure the electric current flowing from/to standard electrode (SHE) and the solution whose red-ox potential should be examined. These potential (or better the potential difference is called Eh e - P H 2 (g) ph 2 = 1 atm Transporte iónico [H + ] = 1 [I - ] = 1 SHE Eléctrodo indicador 1/2H 2 (g) H + + e - 1/2 I 2 (aq) + e - I -

5 Eh-pH conditions in natural waters Eh AMD Water liberates O 2 (g) E = ph Rain Lagoons River Oceans 0 E= ph Swamps. Marine sed Water liberates H 2 (g) ph

6 Eh-pH diagram for the most common dissolved S species HSO S Eh H 2 S SO HS ph

7 Sulfides(1) FeS 2 pyrite, (marcasite ) FeS - mackinawite, greigite, hidrotroilite, and other acid volatile sulfides (AVS) of MeS type, of the following elements: Ag, Fe, Cd, Hg, Mn, Te, Se, As, Pb, Sb, Co, Ni, Mo, etc S 8, elemental

8 Sulfides (2) Sulfide minerals are usually operationally partitioned into two biogeochemical pools: HCL soluble S: acid volatile sulfide, mainly FeS (AVS) Cr(II)-reducible S: mainly pyrite and galena (CRS)

9 Evaporites CaSO 4 : anhydrite CaSO 4 2H 2 O: gypsum Sulfates Biogenics: SrSO 4 : celestite, secretioned by acantarias (pelágic protozoarians, Radiolarians) BaSO 4 : barite

10 S- Biogases Hydrogen Sulfide (H 2 S) Carbon disulfide (CS 2 ) Carbonyl Sulfide (COS) Methyl mercaptan (CH 3 SH) Dimethyl sulfide (DMS: CH 3 SCH 3 ) Dimethyl disulfide (DMDS: CH 3 SSCH 3 )

11 Organic Sulfur Amino Ácidos: DMS Metionina: CH 3 SCH 2 CH 2 CH(NH 2 )COOH Cisteina: HSCH 2 CH(NH 2 )COOH mercaptan S-linkage among lipids and proteins Functional groups in lipids: often formed during organic matter transformation: natural vulcanisation

12 Sulfate Concentrations μm mg/l Seawater Natural River Actual River Continental Rain Marine Rain

13 Sulfate in Rain: sources Sea-salt sulfate Anthropogenic SO 2 Biogenic reduced S gases Volcanic emissions Forest Burning Soil dust Plant aerosols

14 Sulfate in Rain: Sea-salt Sulfate derived from sea-spray Sea-salt sulfate= sulfate/cl ratio in seawater * Cl in rain Total flux: 44 Tg S/yr 4 Tg to land 40 Tg redeposited at sea

15 Cl in rain : mostly marine origin

16 Sulfate in rain: anthropogenic emissions ofso 2 Combustion of organic-s and associated pyrite by humans (coal, petrol) releases SO 2 to atmosphere SO 2 + OH-radicals or H 2 O 2 gives H 2 SO 4 H 2 SO 4 dissociates into 2H + + SO 2-4 Major contributor to Acid Rain

17 Total wet deposition of SO 4 (mmol/m2/yr)

18 Excess S deposition (kg S/ha/yr)

19 Sulfate in Rain: Biogenic reduced S Natural source derived from oxidation of reduced S-containing biogases (e.g): H 2 S + OH SO 2 (CH 3 ) 2 S + OH SO 2 Biogas sources: open ocean marine coastal areas anoxic, high-organic soils oxic, soils

20 Relative S sources to rivers Atmospheric salt: 2 % Evaporite weathering: 22 % Natural atmospheric input: 17% Volcanism: 5% Pyrite weathering: 11% Pollution: 44 %

21 Global Sulfur Cycle all values in g S/yr Wet and dry deposition 84 Transport to sea Dust Human mining and extraction Natural weathering and erosion Biogenic gases Rivers 213 Deposition Sea salt Transport to land 43 Biogenic gases Pyrite Hydrothermal sulfides 1 0 Schlesinger W.H. 1997

22 Sulfur-Climate interactions: the ocean emissions Some algae (Emiliani huxleyi, Phaeocystis) contain DMSP (dimethylsulphoniopropionate) as an osmolyte to alleviate salt stress and prevent freezing. DMSP is converted to DMS (dimethylsulfide) DMS is either oxidised microbially in water to DMSO (dimethylsulfoxide) or exchanged to atmosphere

23 Emiliania huxleyi

24 Emiliania blooms (Seawifs)

25 DMS and climate

26 Sedimentary S cycle Sulfate reduction Sulfide precipitation reactions Sulfide oxidation Disproportionation reactions

27 Sulfate reduction (1) Assimilatory sulfate reduction for the biosynthesis of organic sulfur compounds Dissimilatory sulfate reduction from which microorganisms (sulfate reducers) obtain energy: 2CH 2 O + SO 2-4 2HCO H 2 S Sulfate reduction coupled to anaerobic methane oxidation CH 4 + SO 4 2- HCO HS - + H 2 O

28 Sulfate reduction (2)

29 Sulfate reduction Oxic mineralization Denitrification Mn-reduction Fe-reduction R SO4-reduction 0 Carbon loading (mmol/m2/yr) 0.6

30 Formation of ferric sulfides

31 Pyrite Formation in Sediments Sulfate reduction (Desulfovibrio desulfuricans): SO (CH 2 O)->2HCO 3- +H 2 S Two step reaction Reaction H 2 S with Fe 2+ or reactive Fe-mineral 4Fe 2 O 3 +9H 2 S ->8FeS+SO H 2 O+2H + Reaction of iron sulfide with elemental sulfur FeS + S 0 -> FeS 2 Recently incomplete oxidation of H 2 S or FeS by O 2, NO 3 -, MnO 2 or FeOOH In strictly anoxic sediments FeS+H2S ->FeS 2 +H 2 In salt sediments Fe 2+ + S 0 ->FeS 2 Schultze/Zabel 2000

32 Fate of sulfide: oxidation (1) Oxidation of sulfide, oxic or anoxic (e.g., with NO 3, FeOOH), chemical or biological is often incomplete: many intermediates and shunts are involved: elemental S thiosulfate sulfite

33 Weathering of sulfides and gossan formation

34 Impacts in drainage bassin the AMD Ferrihid rite 4FeS H 2 O + 15 O 2 4Fe(OH) SO H + FeS Fe H 2 O = 15Fe2+ + 2SO H + Essentially bacterially mediated by Thiobaccilus ferrooxidans

35 FeS 2 (s) + 7/2 O 2 + H 2 O = Fe SO H + Jarosite NaFe 3 (SO 4 ) 2 (OH) 6

36 Sulfur valency compounds and redox reactions

37 Disproportionation of S Bacterial disproportionation is a fermentation process whereby sulfate and sulfide are produced from a sulfur compound with intermediate redox state. Profit from the energy within intermediate S compounds. Examples: Sulfite to one sulfide and three sulfate 4HSO - 3 H 2 S + 3SO H + Thiosulfate to one sulfide and one sulfate S 2 O 2-3 H 2 S + SO 2-4 Elemental S to three sulfide and one sulfate 4 S + 2H 2 O + 2OH - 3H 2 S + SO 2-4

38 S oxidation and autotrophy Often more than 90 % of sulfide generated is re-oxidized so that energy is recycled again. Chemolithotrophs conserve this energy: photoautotrophs : purple and green sulfur bacteria (Chromatium, Chlorobium) chemolithoautotroph: colorless sulfur bacteria (Thiobacillus, Beggiatoa)