The Sulfur Cycle

Sulfur exists naturally in several oxidation states, and its participation in oxidation-reduction reactions has important geochemical consequences. For example, when an extremely insoluble material, FeS2, is precipitated from seawater under conditions of bacterial reduction, Fe and S may be sequestered in sediments for periods of hundreds of millions of years. Sulfur can be liberated biologically or volcanically with the release of H2S or SO2 as gases. 

There are nine known isotopes of sulfur of which four are stable  

The prevalence of sulfur s second most abundant isotope, S, along with the fractionation known to occur in many biogoechemical processes, make isotopic studies of sulfur a potentially fruitful method of unraveling its sources and sinks within a given reservoir. 

With the exception of ionic sulfides formed from highly electropositive elements (i.e., Na, K, Ca, Mg), sulfur bonding in natural environments is covalent. When fully oxidized, however, the covalently bonded sulfur atom exists 

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The sea bed functions as a giant anaerobic reactor where element cycles are coupled in a way that differs fundamentally from the cycles in the oxic ocean. The microbiological and geochemical processes of sulfur transformation thereby play key 

Rather than being a simple cycle, composed of anaerobic bacterial reduction of sulfate to hydrogen sulfide and aerobic reoxidation of H S to SO, the transformations of sulfur in aquatic sediments include a combination of intermediate cycles or shunts and interactions with other element cycles (e.g., Jorgensen 1990 Luther and Church 1991 Thamdrup et al. 1993 van Cappellen and Wang 

By bacterial sulfate reduction H S is produced as the extracellular end-product (Widdel and Hansen 1991). During the oxidation of H S, oxic or anoxic, chemical or biological, compounds such as zero-valent sulfur (in elemental sulfur, polysulfides, or polythionates), thiosulfate (S Oj ), and sulfite (SOj ) are produced (Cline and Richards 1969 Pyzik and Sommer 1981 Kelly 1988 Dos Santos Afonso and Stumm 1992). These intermediates may then be further transformed by one or several of the following processes  

By bacterial disproportionation H S and are produced concurrently without participation of an external electron acceptor or donor (Bak and Pfennig 1987 Thamdrup et al. 1993). The biogeo-chemical transformations of sulfur in marine sediments are closely coupled to the cycles of iron and manganese. Sulfate, iron oxides, and manganese oxides all serve as electron acceptors in the respiratory degradation of organic matter. As there are also non-enzymatic reactions between iron, manganese and H S within the sediment, the quantification of dissimilatoiy, heterotrophic Fe and Mn reduction is particularly difficult. 

The precipitation of (authigenic) iron sulfides resulting from the reaction between H S and Fe phases exerts an important control on the distribution of HjS in marine pore waters (Goldhaber and Kaplan 1974 Canfield 1989 Canfield et al. 

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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. 

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 ... 

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-... 

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. 

Jorgensen, B. B. (1990). A thiosulfate shunt in the sulfur cycle of marine sediments. Science 249, 152-154. 

Kertesz MA (1999) Riding the sulfur cycle—metaholism of sulfonates and sulfate esters hy Gram-negative bacteria. FEMS Microbiol Revs 24 135-175. 

Fitzgerald JW (1976) Sulfate ester formation and hydrolysis a potentially important yet often ignored aspect of the sulfur cycle of aerobic soils. Bacterial Rev 40 698-721. 

Determinations of 8 " S have been extensively used in studies on the sulfur cycle, including reactions involving microbial anaerobic reduction of sulfate and thiosulfate (Smock et al. 

A great number of processes and sinks related to the sulfur cycle in a sewer affect to what extent hydrogen sulfide is an odor problem. Figure 4.4 outlines the major pathways that also will be major subjects for detailed descriptions in Chapter 6. Although not all aspects depicted in Figure 4.4 can be easily quantified, they should be included in an evaluation of odor problems associated with sewage transport. 

The transformation and transport rates that are involved in the sulfur cycle shown in Figure 4.4 determine to what extent the relevant components will exist in the different phases of the sewer system. As already shown — and further focused on when dealing with the concrete corrosion in Section 6.2.6 — the... 

FIGURE 4.4. Main pathways and sinks for the sulfur cycle in a sewer network associated with odor problems. 

From a general point of view, but still related to sewer conditions, the anaerobic processes in wastewater are dealt with in Chapter 3, especially in Section 3.2.2. In the following, the sulfur cycle is focused on. A part of this cycle proceeds under anaerobic conditions, and another part is aerobic. In a sewer system with changing aerobic and anaerobic conditions, this combined cycle is of particular interest but, at the same time, also complex to deal with. The nature of the sulfur cycle in a sewer is further complicated because the processes proceed in and between the biofilm, the sewer sediments, the water phase, and the sewer atmosphere. 

Details of the flow shown in Figure 4.4 are depicted in Figure 6.1 that shows the biological transformations related to the sulfur cycle. 

Anaerobic processes in wastewater of sewer systems in terms of both the organic matter transformations and the sulfur cycle have been dealt with in Chapter 6. Particularly, Section 6.4 has focused on the integrated aerobic-anaerobic sewer process model. From a conceptual point of view, the anaerobic... 

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