Oxidation of Sulfide and Elemental Sulfur

Although sulfide (mainly H2S) may be oxidized nonenzymatically to elemental sulfur by giving its electrons to ferricytochrome c, sulfide-cytochrome c 

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Oxidation of sulfide and elemental sulfur (S) and related compounds to sulfate (SO4-). 

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. 

As this short discussion shows, the kinetics of formation of the single parameters (Fe2+ and H2S) may control the extent and the pathway of pyrite formation. Oxidation of sulfide by elemental sulfur to form poly sulfides (pathway 1) should predominate at the oxygen-sulfide interface of very productive... 

The oxidation and reduction of elemental sulfur and sulfide occur in different species of bacteria, e.g., the oxidation of sulfides via elemental sulfur to sulfate takes place in Chromaiia, the alternative oxidation to sulfate in Thiobacillus. The reduction of sulfate to sulfide occurs in Desiilfovibrio, The biosynthesis of organic sulfur compounds from sulfate takes place mainly in plants and bacteria, and the oxidation of these compounds to sulfate is characteristic of animal species and of heterotrophic bacteria. 

In agreement with the statements of Trueper (1) one can say that principally different dissimilatory sulfur metabolic pathways exist in Anoxyphotobacteria for the oxidation of sulfite to sulfate (via APS or directly), the utilization of thiosulfate (splitting or formation of tetrathionate), and the oxidation of sulfide or elemental sulfur (by a "reverse" siroheme sulfite reductase or other mechanisms). 

FCSD is a periplasmic enzyme found in a number of phototrophic bacteria, as well as in Paracoccus denitrificans, that catalyzes the oxidation of sulfide to elemental sulfur (Cusanovich et al., 1991 Wodara et al., 1997). FCSD from Chromatium vinosum is a 67kDa heterodimer consisting of a 46kDa flavoprotein subunit and a 21 kDa diheme cytochrome. The secondary electron acceptor is probably a cytochrome (Gray and Knaff, 1982). The FAD is bound covalently to the flavoprotein subunit via an 8-a-methyl(S-cysteinyl) thioether linkage. 

The use of Thiobacillus species has been studied quite extensively. Sublette and Sylvester especially focused on the use of Thiobacillus denitrificans [50-52] for aerobic or anaerobic oxidation of sulfide to sulfate. In the anaerobic oxidation NOs was used as an oxidant instead of oxygen (confirm Table 2). Buisman used a mixed culture of Thiobacilli for the aerobic oxidation of sulfide to elemental sulfur and studied technological applications [53-55]. Visser showed the dominant organism in this mixed culture to be a new organism named Thiobacillus sp. W5 [6]. 

A summary of the available results on the extent of isotope fractionation during sulfide oxidation is summarized in Table 3. The phototrophic oxidations of sulfide to elemental sulfur and of elemental sulfur to sulfate yield only small or negligible fractionations. Small fractionations also accompany the non-phototrophic, biologically-mediated, oxidation of sulfide to elemental sulfur, as well as the oxidation of sulfur intermediate compounds to sulfate (Table 3). However, significant depletion of sulfate in... 

Disproportionation reactions do not cause a net oxidation of the sulfur species, yet they have a key function in sulfide oxidation. Disproportionation provides a shunt in the sulfur cycle whereby the H S formed by this reaction may be oxidized again to the same sulfur intermediate by metal oxides. Manganese oxide, for example, rapidly oxidizes H S to S without participation of bacteria, but does not oxidize the S further to sulfate (Burdige 1993). The elemental sulfur may, however, be disproportionated (Eq. 8.19) whereby a fourth of it is oxidized completely to sulfate while the remaining three fourths return to the sulfide pool. Through repeated partial oxidation of sulfide to elemental sulfur with manganese oxide and subsequent disproportionation of the elemental sulfur to sulfate and sulfide a complete oxidation of sulfide to sulfate by manganese oxide may be achieved (Fig. 8.16 Thamdrup et al. 1993 Bdttcher and Thamdmp 2001) ... 

Whitcomb, J. H., R. D. DeLaune, and W. H. Patrick, Jr. 1989. Chemical oxidation of sulfide to elemental sulfur its possible role in marsh energy flow. Marine Chem. 26 205-214. 

Sulfur/Sulfide Oxidizing Bacteria This broad family of aerobic bacteria derives energy from the oxidation of sulfide or elemental sulfur to sulfate (Fig. 10.10). Some types of aerobes can oxidize sulfur to sulfuric acid, with pH values as low as one reported. These Thiobacillus strains are most commonly found in mineral deposits, and are largely responsible for acid mine drainage, which has become an environmental concern. They proliferate inside sewer lines and can cause rapid deterioration of concrete mains and the reinforcing steel therein. 

Cupric sulfide, copper(II) sulfide, reacts with hot nitric acid to produce nitric oxide gas, NO, and elemental sulfur. Only the oxidation numbers of S and N change. Write the balanced equation for the reaction. 

DOXOSULFREEN A process for oxidizing hydrogen sulfide to elemental sulfur, based on the SULFREEN process and catalyzed by copper on alumina. Use of a fairly low temperature (90 to 140°) minimizes the further oxidation to S02. Developed by Elf and Lurgi. 

Evidence for the existence of PoS is much stronger. Aqueous solutions in HCl containing Po and Po yield a precipitate of PoS. During the course of this reaction, Po is reduced to Po with the concurrent oxidation of sulfide to free sulfur. The same sulfide can be prepared by the reaction between polonium hydroxide and ammonium sulfide. The compound has not been successfully prepared by the direct reaction between the elements. 

Members of the Sulfolobaceae (Sulfolobus and Acidianus) are facultative heterotrophs that can oxidize hydrogen sulfide to elemental sulfur and the latter to sulfuric acid. Nothing is known about the enzymology of these processes and to what extent membrane-boimd enzymes are involved. What information there is relates to the electron transport system of Sulfolobus which is relatively simple consisting as it does of dehydrogenases for succinate and NADH, a quinone pool, a complex of b-cytochromes, and several oxidases. 

Coal mining exposes suffides (primarily pyrite) in coal and associated rocks to oxygen and moisture. These oxidize the sulfides and form sulfuric acid. The resulting acidic waters (referred to as acid mine drainage (AMD)) adversely impact the biota in watersheds downstream from active and abandoned mines. Oxidation of the sulfides also releases chalcophyllic trace elements into the water. Many of these elements precipitate in oxygenated surface waters and are concentrated in stream sediments (Goldhaber et at, 2001). 

While some reduced sulfur, either of biogenic or non-biogenic origin, may accumulate in the environment as metal sulfides and elemental sulfur, or be incorporated into fossil organic matter, most is eventually oxidized to sulfate, a process in which microbial activities play a major role. 

Figure 9.3-5 is a Pourbaix. predominant-area, diagram for the S-O-HjO system for a total activity of all dissolved sulftir species of 10 1 ( S — 10" ). The only stable sulfiir species are HSOJ. SO2-. H.S. HS". and elemental sulfur. The formation of elemental sulftir films occurs in acid solutions as indicated. In basic solution, during the oxidation of sulfur-bearing compounds, intermediate metastabte solfor species such as thiosulfate, dithionate. and polythionates form. This is a problem previously described in the Sherrill Gordon process. Under acid conditions, during the dissolution of sulfide minerals, elemental sulfur layers often form but metastable solfor intermediates such as thiosulfate and sulfite are not observed. 

Studies of isotope fractionation during sulfide oxidation are rather scant and important sulfide-oxidizing organisms like, for example, Beggiatoa sp. and Thioploca sp., have yet to be studied. Nevertheless, the available results suggest that biologically mediated oxidation of sulfate to elemental sulfur and sulfate lead to only minimal isotope fractionation. 

In the Maumee Chemical [60] plan for saccharin, there is a curiosity with respect to the oxidation number of the sulfur atom in the source material as shown in step 1 of the following scheme. It conld be either sodium sulfide (oxidation number = -2) or elemental sulfur (oxidation number = 0). In this case, the arithmetic mean of these valnes was nsed as the oxidation number of the sulfur atom in the starting material (oxidation number = -1) which, by the way, corresponds to the oxidation number of sulfur in disodium disulfide that is generated in situ from sodium sulfide and elemental sulfur. 

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