Sulfur-cycles

Sulfur, a reactive element with stable oxidation states ranging from —2 to +6, is among the 10 most abundant elements in the Earth s crust. In living organisms, sulfur occurs mainly as sulfhydryl groups in amino acids and their polymers. At an approx- 

In its fully oxidized state, sulfur exists as sulfate. Sulfate is the second most abundant anion in seawater, and the SO4- in marine environments represents a large, slowly cycled sulfur reservoir. 

Living and dead organic matter compose a smaller, more rapidly cycled sulfur reservoir. Largely inert sulfur reservoirs include metal sulfides in rocks, elemental sulfur deposits, and fossil fuels. 

Organosulfur decomposition in soils and sediments yields mercaptans and H2S. Analogous to ammoni-fication, this process is referred to as desulfurization. 

The H2S and NH3 are released into the atmosphere, contributing to the characteristic smell associated with putrefaction. 

Sulfur occurs abundantly in evaporite (5 x 10 g), sediments (mainly shale, 

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Many problems have been reported (163), and the process has been abandoned because of the difficulty in handling sohds. Processes which are thought to have the best likelihood of success ate based on sulfuric acid decomposition. Three prominent cycles are based on this reaction the General Atomics iodine—sulfur cycle... 

The harmful effects of air pollutants on human beings have been the major reason for efforts to understand and control their sources. During the past two decades, research on acidic deposition on water-based ecosystems has helped to reemphasize the importance of air pollutants in other receptors, such as soil-based ecosystems (1). When discussing the impact of air pollutants on ecosystems, the matter of scale becomes important. We will discuss three examples of elements which interact with air, water, and soil media on different geographic scales. These are the carbon cycle on a global scale, the sulfur cycle on a regional scale, and the fluoride cycle on a local scale. 

Biogeochemistry- Study of microbially mediated chemical transformations of geochemical interest, such as nitrogen or sulfur cycling. 

One of the things that environmental scientists do is to keep track of important elements in the biosphere—in what form do these elements normally occur, to what are they transformed, and how are they returned to their normal state Careful studies have given clear, although complicated, pictures of the "nitrogen cycle," the "sulfur cycle," and the "phosphorus cycle," for example. The "carbon cycle," begins and ends with atmospheric carbon dioxide. It can be represented in an abbreviated form as ... 

R. W. FairbriiXjE, Encyclopedia of Geochemistry and Environmental Sciences, Van Nostrand, New York, 1972.. See sections on Geochemical Classification of the Elements Sulfates Sulfate Reduction-Microbial Sulfides Sulfosalts Sulfur Sulfur Cycle Sulfur Isotope Fractionation in Biological Processes, etc., pp. 1123 - 58. 

The gloabal geochemical sulfur cycle has been extensively studied In recent years for both commercial and environmental reasons. 

M. V. Ivanov and J. R. Feienet (etls.), The Global Bio-geochemical Sulfur Cycle, SCOPE Report 19, Wiley, Chichester, 198. 495 pp,... 

P. Brimri.ecombe and A. Y. Lein (eds.), E oluiion of the Global Biogeochemical Sulfur Cycle, SCOPE Report 39, Wiley, Chichester. 1989, 276 pp. 

It is often taken for granted that the oxygen content of the air is nearly constant at ca. 20% of the atmospheric volume, that most of the liquid water on the planet is aerobic (i.e. contains O2), and that most water has pH values relatively close to neutral" (close to 7). However, these circumstances are not mere coincidences but are in fact consequences of the interaction of key global biogeochemical cycles. For instance, the pH of rainwater is often determined by the relative amounts of ammonia and sulfuric acid cycled through the atmosphere, a clear example of interaction between the nitrogen and sulfur cycles. 

Fig. 4-13 Calculated and observed annual wet deposition of sulfur in mgS/m per year. (Reprinted from "Atmospheric Environment," Volume 30, Feichter, J., Kjellstrom, E., Rodhe, H., Dentener, F., Lelieveld, and Roelofs, G.-J., Simulation of the tropospheric sulfur cycle in a global climate model, pp. 1693-1707, Copyright 1996, with permission from Elsevier Science.)...

Another major process at the Earth s surface not involving rapid exchange is the chemical weathering of rocks and dissolution of exposed minerals. In some instances the key weathering reactant is H30 in rainwater (often associated with the atmospheric sulfur cycle), while in other cases H30" comes from high concentrations of CO2, e.g., in vegetated soils. 

Let us turn now to a detailed, box-model investigation of a regional sulfur cycle. The discussion so far suggests that the sulfur cycle over much of the ocean should be largely unin-... 

Figure 13-5 is the box model of the remote marine sulfur cycle that results from these assumptions. Many different data sets are displayed (and compared) as follows. Each box shows a measured concentration and an estimated residence time for a particular species. Fluxes adjoining a box are calculated from these two pieces of information using the simple formula, S-M/x. The flux of DMS out of the ocean surface and of nss-SOl back to the ocean surface are also quantities estimated from measurements. These are converted from surface to volume fluxes (i.e., from /ig S/(m h) to ng S/(m h)) by assuming the effective scale height of the atmosphere is 2.5 km (which corresponds to a reasonable thickness of the marine planetary boundary layer, within which most precipitation and sulfur cycling should take place). Finally, other data are used to estimate the factors for partitioning oxidized DMS between the MSA and SO2 boxes, for SO2 between dry deposition and oxidation to sulfate, and for nss-SO4 between wet and dry deposition. 

Comparison of Figs 13-6a and 13-6b clearly demonstrates the degree to which human activity has modified the cycle of sulfur, largely via an atmospheric pathway. The influence of this perturbation can be inferred, and in some cases measured, in reservoirs that are very distant from industrial activity. Ivanov (1983) estimates that the flux of sulfur down the Earth s rivers to the ocean has roughly doubled due to human activity. Included in Table 13-2 and Fig. 13-6 are fluxes to the hydrosphere and lithosphere, which leads us to these other important parts of the sulfur cycle. 

Fig. 13-6 Major fluxes of the global biogeochemical sulfur cycle excluding (a) and including (b) human activity (modified from Ivanov, 1983). Numbers in circles designate fluxes described in Table 13-2.

Friend,. P. (1973). The global sulfur cycle. In Chemistry of the Lower Atmosphere" (S. I. Rasool, ed.). Plenum Press, New York. 

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