The sulphur cycle

A small part of HjS in the biosphere remains in the form of insoluble sulphides or is spontaneously oxidized with oxygen to elementary sulphur. Photosynthetic and chemoautotrophic bacteria are capable of oxidizing HjS and elementary sulphur to sulphate. This biological oxidation is aflFected either aerobically (colourless sulphur bacteria) or anaerobically (photosynthetic purple and green sulphur bacteria). 

The aerobic sulphur bacteria Thiobacillus thiooxidans and T. thioparus are obligate chemoautotrophs. They occur in soils, muds, lakes and oceans and use the oxidation of sulphur and sulphur compounds as their energy 

The photosynthetic species, Chlorobium and Chromatium, develop under anaerobic conditions when light is available to provide the source of energy for growth. H2S is used as a reductant in converting CO2 to their cell material. During photosynthesis in muds and lake waters they may oxidize the sulphide and precipitate sulphur on such a scale that it can be ecomomically mined. 

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Freney, J. R., Ivanov, M. V., and Rodhe, H. (1983). The sulphur cycle. In The Major Biogeochemical Cycles and Their Interactions, SCOPE 21" (B. Bolin and R. B. Cook, eds). WUey, Chichester. 

Jorgensen BB. 1982. Ecology of the bacteria of the sulphur cycle with special reference to anoxic-oxic interface environments. Philos Trans R Soc Lond B Biol Sci 298 543-561. 

It illustrates the different stages in the manufacture of soda by the Leblanc process. The sulphur cycle in that process is also illustrated diagrammatically by the scheme ... 

Over the past few years we have been studying the waters around the United Kingdom, including the North Sea, Irish Sea and N.E. Atlantic, in order to characterise dimethyl sulphide (DMS) emissions and assess the significance of this natural contribution to acidity of rainfall and the sulphur cycle. Biogenic DMS concentrations in seawater vary considerably both temporally and spatially and coastal and shelf water systems often contain higher concentrations of volatile sulphur than the open oceans (1.2). 

Session 4 focused on recent advances in the thermochemical copper chloride and calcium bromide cycles. Much of the current research on thermochemical cycles for hydrogen production involves the sulphur cycles (sulphur-iodine, hybrid sulphur), however, these cycles require very high temperatures ( 800-900°C) to drive the acid decomposition step. The interest in the Cu-Cl and Ca-Br cycles is due to the lower peak temperature requirements of these cycles. The peak temperature requirement for the Cu-Cl cycle is about 550°C, which would allow this cycle to be used with lower temperature reactors, such as sodium- or lead-cooled reactors, or possibly supercritical water reactors. Ca-Br requires peak temperatures of about 760°C. Both of these cycles are projected to have good efficiencies, in the range of 40%. Work on Cu-Cl is ongoing in France, Canada and the United States. Work on Ca-Br has been done primarily in Japan and the US, with the more recent work being done in the US at ANL. The papers presented in this session summarised the recent advances in these cycles. 

Holt B. D. and Kumar R. (1991) Oxygen isotope fractionation for understanding the sulphur cycle. In Stable Isotopes Natural and Anthropogenic Sulphur in the Environment, SCOPE 43 (Scientific Committee on Problems of the Environment) (eds. H. R. Krouse and V. A. Grinenko). Wiley, pp. 27-41. 

Habicht K. S. and Canfield D. E. (1996) Sulphur isotope fractionation in modern microbial mats and the evolution of the sulphur cycle. Nature 382(6589), 342-343. 

J0rgensen B. B. (1988) Ecology of the sulphur cycle oxidative pathways in sediments. In The Nitrogen and Sulphur Cycles (eds. J. A. Cole and S. J. Eerguson). Cambridge University Press, Cambridge, pp. 31—63. 

Postgate, J.R., 1968. The sulphur cycle. In G. Nickless (Editor), Inorganic Sulphur Chemistry. Elsevier, Amsterdam, pp. 259—279. 

Farquhar et al. (2000) investigated mass-independent fractionation in sulphur isotopes from across the period when oxygen levels are thought to have risen. They found a major change somewhere between 2090 and 2450 Ma ago. In rocks older than this, gas-phase atmospheric reactions may have influenced the sulphur cycle, playing a role in determining the oxidation state of sulphur. This would imply that atmospheric oxygen had low partial pressures, and microbial oxidation and reduction of sulphur were... 

Parts of the sulphur cycle which are thought to have changed significantly as a result of human activities include the following ... 

Fig. 7.17 Simplified version of the sulphur cycle (after Brimblecombe et al. 1989). (a) Sulphur cycle as it is thought to have been prior to any major anthropogenic influence, (b) Sulphur cycle as it was in the mid-1980s. Units for inter-reservoir flows are in TgSyU1 (i.e. 1012gSyr 1). With permission from the Scientific Community on Problems of the Environment—SCOPE, John Wiley Sons Ltd.

A summary of the sulphur cycle is shown in Fig. 3.19, incorporating important geological pathways as well as major biologically mediated processes. In organisms thiol groups are the dominant form of sulphur, as in the amino acids cysteine and methionine (see Fig. 

Fig. 3.19 Summary of the sulphur cycle (oxidation states of sulphur are shown in parentheses). Solid lines show major microbial pathways broken lines show geological processes involving sedimentary organic-S, calcium sulphate (gypsum/anhydrite) andiron(II) sulphide (pyrite).

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