Reduced sulfur species, oxidation

Se,o,s Sulfur isotopic composition of oxidized sulfur species. S S. r.s Sulfur isotopic composition of reduced sulfur species. 8 Stotaf Total sulfur isotopic composition of aqueous sulfur species. 

By giving the values of temperature, ES, ionic strength, FeS content of sphalerite and Ag content of electrum, we can place a limit on fo2- However, we cannot know whether /02 lies in the predominance field of reduced sulfur species or that of oxidized sulfur species from the constraints mentioned above. 

The /02 of ore fluids responsible for the epithermal base-metal veins might have been in the predominance field of reduced sulfur species because (1) pyrrhotite is occasionally found in these deposits, (2) selenium content of argentite is very low and (3) H2S is dominant in the present-day epithermal base-metal fluids. Implication of selenium content of sulfides will be considered later. Barite is sometimes found in the late-stage of mineralization. Thus, it is likely that /oj of barite stage lies in the predominance field of oxidized sulfur species. 

On the other hand, the ore fluids responsible for epithermal Au-Ag vein-type deposits contain appreciable amounts of oxidized sulfur species, together with reduced sulfur species. Oxidized sulfur species/reduced sulfur species ratio is considered to be greater than 1, that is, /02 lies in the predominance field of oxidized sulfur species. The reasons for this estimation are (1) hematite is common in the deposits, (2) barite is found... 

The wide variation in the selenium content of acanthite from epithermal Au-Ag vein-type deposits can be explained by assuming that the acanthite formed within the predominance region of oxidized sulfur species at a constant ESe/ES ratio, ionic strength, temperature and pH (Figs. 1.93 and 1.94). The very low selenium content of acanthite from epithermal Pb-Zn vein-type deposits suggests the predominance region of reduced sulfur species (Figs. 1.93 and 1.94). 

This mechanism as a main cause for epithermal-type Au deposition is supported by sulfur isotopic data on sulfides. Shikazono and Shimazaki (1985) determined sulfur isotopic compositions of sulfide minerals from the Zn-Pb and Au-Ag veins of the Yatani deposits which occur in the Green tuff region. The values for Zn-Pb veins and Au-Ag veins are ca. +0.5%o to -f4.5%o and ca. -l-3%o to - -6%c, respectively (Fig. 1.126). This difference in of Zn-Pb veins and Au-Ag veins is difficult to explain by the equilibrium isotopic fractionation between aqueous reduced sulfur species and oxidized sulfur species at the site of ore deposition. The non-equilibrium rapid mixing of H2S-rich fluid (deep fluid) with SO -rich acid fluid (shallow fluid) is the most likely process for the cause of this difference (Fig. 1.127). This fluids mixing can also explain the higher oxidation state of Au-Ag ore fluid and lower oxidation state of Zn-Pb ore fluid. Deposition of gold occurs by this mechanism but not by oxidation of H2S-rich fluid. 

Figure 1.127. (a) Schematic representation of change in SS04 "/EH2S ratio (concentration ratio of oxidized sulfur species and reduced sulfur species) and value of reduced sulfur species (8 Sr.s.s.) in ore... 

The second type of biological process is a direct treatment process, in which bacteria oxidize the sulfur species. This oxidative desulfurization reaction is carried out in the presence of an electron acceptor (such as nitrate, NOs ), a source of carbon (such as carbon dioxide or I ICO, ), and a source of reduced nitrogen (such as ammonia). The... 

In addition to these nuclear reactions, myriads of other gas-phase transformations produce low-vapor pressure species, with the oxidation of SO2 and other reduced sulfur species dominating aerosol formation and growth. Oxidation of SO2 in the gas phase produces H2SO4, a readily condensable species that either combines with other molecules (new particle formation) or condenses on existing aerosols. 

We can check our results for the Red Sea brine against two independent pieces of information. In our results, sulfate species such as NaSO dominate reduced sulfur species such as H2S(aq) and HS-, in seeming accord with the failure of analysis to detect reduced sulfur in the brine. The predominance of sulfate over sulfide species in our calculation reflects the oxidation state resulting from our assumption of equilibrium with sphalerite. 

The most notable feature of the sulfur isotope geochemistry of lunar rocks is the uniformity of 8 " S-values and their proximity to the Canyon Diablo standard. The range of published 8 " S-values is between -2 to +2.5%o. However, as noted by Des Marais (1983), the actual range is likely to be considerably narrower than 4.5%o due to systematic discrepancies either between laboratories or between analytical procedures. The very small variation in sulfur isotope composition supports the idea that the very low oxygen fugacities on the Moon prevent the formation of SO2 or sulfates, thus eliminate exchange reactions between oxidized and reduced sulfur species. 

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). 

Reduced sulfur compounds are ubiquitous in aqueous and atmospheric systems (10,11). Natural sources of reduced sulfur species in aqueous environment result from biological reduction of sulfate, anaerobic microbial processes in sewage systems, putrefaction of sulfur-containing amino-acids (12), oxidative decomposition of pyrite (13), and activities of marine organisms in the upper layers of the ocean (14,15). The build-up of sulfides in areas such as the Black Sea is also giving cause for concern (8). 

Oxidized sulfur species occurring in natural waters (sulfate, sulfite, thiosulfate] do not interact with the platinum electrode when in the presence of H2S and the pH-Eh-E52- relations found were similar to the above relations. Thus, the unambiguous relations found between pH, Eh and E22- in aqueous solutions of hydrogen sulfide can be employed to characterize solutions and water samples where hydrogen sulfide is the only reduced sulfur species present. 

In the inorganic world, sulfur would have been available in a variety of oxidation states. Even in a reduced atmosphere, transient SO would have been present from volcanic sources, supplemented by interaction between sulfur-bearing aerosols and oxidants produced by photolytic chemistry in the early UV flux, or from escape of hydrogen to space. Reduced sulfur species would have been widely available in lavas and volcanic vents. Thus, for the early organisms, shuffling sulfur between various oxidation states would have been the best way of exploiting redox ratchets. 

In other circumstances, the oxidation reactions may not proceed to completion. Partial oxidation may result in only a small proportion of the sulfur being transformed to sulfate, with the remainder accumulating as reduced sulfur species (polysulfides and elemental sulfur) at the mineral surface (Janzen et al., 2000) ... 

The reversibility of this phenomenon indicates that during high temperature reduction a reduced sulfur species is formed which migrates to the Pt surface and poisons the Pt surface. Subsequent oxygen treatment oxidizes this sulfur species and the oxidized species migrates back to the support. The oxidized species do not leave the catalyst since the reduction - oxidation sequence produces the same effects over several cycles (see Runs 6 to 9, Table III). 

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... 

Oxidation of thiosulfate also produces small amounts of trithionate (SjOl ), tetrathionate (S4OI"), and pentathionate (SsOl ) (Goldhaber and Kaplan 1974). Summarized in Fig. 12.17 are possible oxidation and disproportionation pathways of reduced sulfur species leading toward sulfate that may be mediated by Thiobacilli. (Disproportionation pathways involve no electron transfer see also O Brien and Birkner 1977 Morse et al. 1987.) More recently, Jorgensen (1990) used radioactive to unravel the complex pathways of sulfide oxidation in sediments. He showed that thiosulfate disproportionation to sulfate and sulfide species... 

Figure 12.17 Possible oxidation pathways for reduced sulfur species to sulfate by Thiobacilli. From M. B. Goldhaber and I. R. Kaplan, The sulfur cycle, in The sea, Vol. 5, Marine chemistry, ed E. D. Goldberg. Copyright 1974 by John Wiley Sons, Inc. Reprinted with permission of John Wiiey Sons, Inc.

---