Oxidation, of reduced sulfur

Connected with the kinetics of oxygen evolution in the early atmosphere is the question of the origin of sulfate, required by the anaerobic sulfate reducers. Did the latter organisms evolve only after oxygen accumulation led to oxidation of reduced sulfur to sulfate This notion was challenged by Peck (1974), who concluded ... 

Tyndall, G. S., and A. R. Ravishankara, Atmospheric Oxidation of Reduced Sulfur Species, Int. J. Chem. Kinet., 23, 483-527 (1991). Urbanski, S. P., R. E. Stickel, and P. H. Wine, Mechanistic and Kinetic Study of the Gas-Phase Reaction of the Hydroxyl Radical with Dimethyl Sulfoxide, / Phys. Chem. A, 102, 10522-10529 (1998). 

Possible sources of sulfate include diffusion from the water column, hydrolysis of sulfate esters, and oxidation of reduced sulfur. Diffusion of S042" into sediments cannot supply sulfate at the measured rates of sulfate reduction. Rates of sulfate diffusion into sediments generally are 2 orders of... 

Finally, there is a potential for inhibition of sulfate reduction by sediment acidification in highly impacted sites. In the first two years of experimental acidification of Little Rock Lake there is no evidence of decreased pH in porewater 1 cm below the interface. It is not clear, however, whether sediment acidification will occur with further increases in acid loadings to the lake. Rudd et al. (fi) showed that porewaters from lakes Hovattn and little Hovattn were acidic at fall turnover and postulated that this may occur by oxidation of reduced sulfur compounds. Although sediments from 223 showed no evidence of acidification after 10 years of experimental lake acidification, the pH of porewater from Lake 114 declined by > 0.5 units after just three years of experimental acidification (fi). 

Atmospheric Considerations. It is probably premature to assess the role of liquid phase oxidation of reduced sulfur compounds in atmospheric chemistry with the limited kinetic data available in the open literature. However, it is appropriate to discuss certain conclusions obvious from the information presented above. 

Environmental Pollution Control. The importance of chemical oxidation of reduced sulfur compounds lies in its application to treatment of wastes-treams from municipal sewage systems, acid mine drainage and industrial plants such as tanneries, paper and pulp mills, oil refineries and textile mills. 

It is clear from the information presented above that liquid phase oxidation of reduced sulfur compounds play an important role in natural processes in water and in wastewater treatments. However, further work is needed to clarify the role of these compounds in cloud chemistry and precipitation acidity. Areas of further investigations should include ... 

The chemoautotrophic fixation of C02 connected with this activity, only minimally contributes to the carbon cycling in most ecosystems. Notable exceptions to this include the deep-sea hydrothermal vent ecosystems, where the whole vent community is supported by the chemoautotrophic oxidation of reduced sulfur, primarily by Beggiatoa, Thiomi-cropira, and other sulfur oxidizers. In environments other than these, the generation of reduced minerals used in chemolithotrophic production is directly tied to the oxidation of photosynthetically produced organic matter. Therefore, sustainable primary production without solar energy input is unthinkable even in the case of chemolithotrophs. 

Oxidation of reduced sulfur species. Oxidation of reduced sulfur species in the presence of oxygen can occur spontaneously, without bacterial mediation. Bacteria of the family Thiobacteriaceae are probably the most important bacteria involved in sulfur oxidation. Of these, bacteria of the genus Thiobacillus have been most studied (Goldhaber and Kaplan 1974 Cullimore 1991). The first product of sulfide oxidation abiotically or by Thiobaccillus is thought to be elemental sulfur according to... 

Evidence was found for the major processes responsible for the acidification of fogwater (i) the scavenging of acidic precursor aerosol, (ii) the scavenging of gaseous nitric acid, and (iii) oxidation of reduced sulfur components to sulfate. Conversion of S02(g) to sulfate in fogwater does not appear to proceed faster than 10% hour" and therefore cannot account for the high acidities observed at the beginning of fog events however, sulfate production in the precursor air parcel can lead to sulfuric acid fog condensation nuclei. 

The chemotrophic (colorless) sulfur bacteria obtain energy from the chemical aerobic oxidation of reduced sulfur compounds. The overall reactions occurring, concerning the biological oxidation of sulfide, are the formation of sulfur (at low oxygen concentrations) and the formation of sulfate (when there is an excess of oxygen) ... 

Hoffmann, M. R., and A. P. Hong (1987), Catalytic Oxidation of Reduced Sulfur Compounds by Homogeneous and Heterogeneous Co(II) Phthalocyanine Complexes, Sci. Total Environ. 64, 99-115. 

Bedard C, Knowles R (1989) Physiology, biochemistry, and specific inhibitors of CH4, NH4+, and CO oxidation by methanotrophs and nitrifiers. Microbiol Rev 53 68-84 Beffa T, Fischer C, Aragno M (1993) Growth and respiratory oxidation of reduced sulfur compounds by intact cells of Thiobacillus novellus (type strain) grown on thiosulfate. Curr Microbiol 26 323-326... 

Ga ago is the discovery of sulfate minerals in deposits of that time (Walter et ah, 1980). Although small amounts of oxygen from abiotic photolysis of water could have resulted in the oxidation of reduced sulfur compounds to form sulfates, it is also possible that part or most of the sulfate was derived from anaerobic photosynthesis according to reaction (2) above. 

In addition to this study, timber samples previously examined by XANES and infused with acid salts have been treated with various biocides to inhibit continued bacterial oxidation of reduced sulfur species. This may also help prevent continuing production of acid. 

Tyndal, G. S.. and Ravishankara, A. R., Atmospheric oxidation of reduced sulfur species. J. Phys. Chem. 23, 483 (1991). 

The pathways of sulfide oxidation in nature are varied, and in fact poorly known, but include (1) the inorganic oxidation of sulfide to sulfate, elemental sulfur, and other intermediate sulfur compounds, (2) the nonphototrophic, biologically-mediated oxidation of sulfide (and elemental sulfur), (3) the phototrophic oxidation of reduced sulfur compounds by a variety of different anoxygenic phototrophic bacteria, and (4) the disproportionation of sulfur compounds with intermediate oxidation states. The first three of these are true sulfide-oxidation pathways requiring either the introduction of an electron acceptor (e g. O2 and NO3 ), or, in the case of phototrophic pathways, the fixation of organic carbon from CO2 to balance the sulfide oxidation. The disproportionation of sulfur intermediate compounds requires no external electron donor or electron acceptor and balances the production of sulfate by the production of sulfide. This process will be taken up in detail in a later section. A cartoon depicting some of the possible steps in the oxidative sulfur cycle is shown in Figure 6. 

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