Oxidation of reduced sulfur compounds

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 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. 

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. 

Formation of elemental sulfur during microbial oxidation of reduced sulfur compounds has been reported [48, 49]. Case histories of corrosion in the presence of elemental sulfur can be attributed to either direct oxidation to H2SO4, or electron transport from the metal through a metal sulfide to elemental sulfur. 

During oxidation of reduced sulfur compounds, more corrosive sulfides are produced under anoxic conditions, causing cathodic reactions. The corrosion rate increases as the reduced and oxidized FeS concentrations increase (Lee et al. 1993a, b). Cathodic depolarization processes also can yield free O2 which reacts with polarized hydrogen on metal surfaces. 

---