The sulfur cycle
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
Oxydation states sulfides transitional sulfates 2-1- 0 2+ 4+ 6+ Dissolved forms H 2 S, HS - S SO 2- SO 2-4, HSO - 8 S 2 O 2-3 3 4 Minerals FeS, MeS FeS 2 CaSO 4 Gases COS, CS 2 SO 2 SO 3
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 -
Eh-pH conditions in natural waters Eh 0.9 0.6 AMD Water liberates O 2 (g) E = 1.22-0.059 ph Rain Lagoons River Oceans 0 E= - 0.059 ph Swamps. Marine sed. -0.6 Water liberates H 2 (g) 2 4 6 8 10 12 14 ph
Eh-pH diagram for the most common dissolved S species 1 0.8 0.6 HSO 4-0.4 0.2 S Eh 0-0.2 H 2 S SO 4 2- -0.4-0.6 HS - -0.8-1 0 2 4 6 8 10 12 14 ph
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
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)
Evaporites CaSO 4 : anhydrite CaSO 4 2H 2 O: gypsum Sulfates Biogenics: SrSO 4 : celestite, secretioned by acantarias (pelágic protozoarians, Radiolarians) BaSO 4 : barite
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 )
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
Sulfate Concentrations μm mg/l Seawater 28900 2712 Natural River 86 8.3 Actual River 115 11 Continental Rain 10-30 1-3 Marine Rain 10-30 1-3
Sulfate in Rain: sources Sea-salt sulfate Anthropogenic SO 2 Biogenic reduced S gases Volcanic emissions Forest Burning Soil dust Plant aerosols
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
Cl in rain : mostly marine origin
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
Total wet deposition of SO 4 (mmol/m2/yr)
Excess S deposition (kg S/ha/yr)
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
Relative S sources to rivers Atmospheric salt: 2 % Evaporite weathering: 22 % Natural atmospheric input: 17% Volcanism: 5% Pyrite weathering: 11% Pollution: 44 %
Global Sulfur Cycle all values in 10 12 g S/yr Wet and dry deposition 84 Transport to sea 81 10 20 Dust 93 22 Human mining and extraction 149 72 Natural weathering and erosion Biogenic gases Rivers 213 Deposition 20 258 144 Sea salt Transport to land 43 Biogenic gases Pyrite 96 39 Hydrothermal sulfides 1 0 Schlesinger W.H. 1997
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
Emiliania huxleyi
Emiliania blooms (Seawifs)
DMS and climate
Sedimentary S cycle Sulfate reduction Sulfide precipitation reactions Sulfide oxidation Disproportionation reactions
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 - 3 + H 2 S Sulfate reduction coupled to anaerobic methane oxidation CH 4 + SO 4 2- HCO 3 - + HS - + H 2 O
Sulfate reduction (2)
Sulfate reduction Oxic mineralization Denitrification Mn-reduction Fe-reduction R SO4-reduction 0 Carbon loading (mmol/m2/yr) 0.6
Formation of ferric sulfides
Pyrite Formation in Sediments Sulfate reduction (Desulfovibrio desulfuricans): SO 4 2- + 2(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 4 2- +8H 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
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
Weathering of sulfides and gossan formation
Impacts in drainage bassin the AMD Ferrihid rite 4FeS 2 + 14 H 2 O + 15 O 2 4Fe(OH) 3 + 8 SO 4 2- + 16 H + FeS 2 + 14Fe 3+ + 8H 2 O = 15Fe2+ + 2SO 4-2 + 16H + Essentially bacterially mediated by Thiobaccilus ferrooxidans
FeS 2 (s) + 7/2 O 2 + H 2 O = Fe 2+ + 2SO 4 2- + 2H + Jarosite NaFe 3 (SO 4 ) 2 (OH) 6
Sulfur valency compounds and redox reactions
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 2-4 + 2H + 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
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)