Sulfide oxidation mechanism

Nicholson R. V. (1994) Iron-sulfide oxidation mechanisms laboratory studies. In The Environmental Geochemistry of Sulfide Mine-wastes (eds. J. L. Jambor and D. W. Blowes). Mineralogical Association of Canada, Nepean, ON, vol. 22, pp. 164-183. 

Nicholson, R. V., 1994, Iron sulfide oxidation mechanisms laboratory studies, in ... 

Most studies of sulfur-fuel oxidation have been performed using hydrogen sulfide, H2S, as the fuel. Consequently, the following material will concentrate on understanding the H2S oxidation mechanism. Much of what is learned from... 

Scheme 6. Reaction mechanism for the photochemical sulfide oxidation reaction.

SCHEME 43. Mechanism of sulfide oxidation catalyzed by a water-modified tartrate-Ti complex. 

Electrochemistry (Continued) purely organic compounds, 342 sulfide oxidation, 361 Electrode materials, 342 Electrophilic allylation, 192 attractive interaction, 196 mechanism, 192, 197 turnover-limiting step, 197 Electroreaction, asymmetric, 342 Electrostatic interaction, 328 Elimination and insertion, 3 Enamide reactions ... 

Measured rates of sulfate reduction can be sustained only if rapid reoxidation of reduced S to sulfate occurs. A variety of mechanisms for oxidation of reduced S under aerobic and anaerobic conditions are known. Existing measurements of sulfide oxidation under aerobic conditions suggest that each known pathway is rapid enough to resupply the sulfate required for sulfate reduction if sulfate is the major end product of the oxidation (Table IV). Clearly, different pathways will be important in different lakes, depending on the depth of the anoxic zone and the availability of light. All measurements of sulfate reduction in intact cores point to the importance of anaerobic reoxidation of sulfide. Little is known about anaerobic oxidation of sulfide in fresh waters. There are no measurements of rates of different pathways, and it is not yet clear whether iron or manganese oxides are the primary electron acceptors. 

In reality, the oxidation of pyrite and other Fe(II) sulfides typically involves several intermediate reactions, which may be enhanced by microbial activity or various chemical species, such as bicarbonate (HCO3-) (Welch et al., 2000 Evangelou, Seta and Holt, 1998). The exact mechanisms of each intermediate reaction are often very complex and poorly understood (Rimstidt and Vaughan, 2003). Mostly likely, sulfide oxidizes in pyrite before iron. Fe(II) is then released into solution as shown by the following reaction involving oxygen and water (Gleisner and Herbert, 2002, 139-140) ... 

Dess-Martin A5-iodane 44 is an extremely useful reagent for the conversion of primary and secondary alcohols to aldehydes and ketones at 25 °C [70]. It does not oxidize aldehydes to carboxylic acids under these conditions. It selectively oxidizes alcohols in the presence of furans, sulfides, and vinyl ethers. The oxidation mechanism involves a facile ligand exchange with alcohols, followed by reductive /1-elimination. 

Studies of the oxidation of organic sulfides with amino acid-derived ligands in acetonitrile revealed very little difference between the mechanism of their oxidation and that of halides, except for one major exception. Despite the fact that acid conditions are still required for the catalytic cycle, hydroxide or an equivalent is not produced in the catalytic cycle, so no proton is consumed [48], As a consequence, there is no requirement for maintenance of acid levels during a catalyzed reaction. Peroxo complexes of vanadium are well known to be potent insulin-mimetic compounds [49,50], Their efficacy arises, at least in part, from an oxidative mechanism that enhances insulin receptor activity, and possibly the activity of other protein tyrosine kinases activity [51]. With peroxovanadates, this is an irreversible function. Apparently, there is no direct effect on the function of the kinase, but rather there is inhibition of protein tyrosine phosphatase activity. The phosphatase regulates kinase activity by dephosphorylating the kinase. Oxidation of an active site thiol in the phosphatase prevents this down-regulation of kinase activity. Presumably, this sulfide oxidation proceeds by the process outlined above. 

As mentioned before, thermal or photolytic treatment of (9-acyl esters (2) generates the corresponding alkyl 2-pyridyl sulfides. Oxidation of the sulfides with raCPBA provides the corresponding sulfoxides, which can be further derived to olefins under heating conditions via Ei mechanism. One example is shown below (eq. 8.8) [26]. 

I. Oxidation mechanisms of sulfide minerals at 25°C. Econ. Geol. 1202-1231 (1960). 

Acidithiobacillus thiooxidans, formerly known as Thiobacillus thiooxidans, is an acidophilic bacterium that oxidizes and thiosulfate, but not iron. Due to the rapid kinetics involved in sulfide oxidation by dissolved oxygen, some sulfide-oxidizing bacteria are in continuous competition with the chemical oxidation mechanism. Acidithiobacillus ferrooxidans, formerly known... 

Sulfide-mineral oxidation by microbial populations has been postulated to proceed via direct or indirect mechanisms (Tributsch and Bennett, 1981a,b Boon and Heijnen, 2001 Fowler, 2001 Sand et al., 2001 Tributsch, 2001). In the direct mechanism, it is assumed that the action taken by the attached cell or bacterium on a metal sulfide will solubilize the mineral surface through direct enzymatic oxidation reactions. The sulfur moiety on the mineral surface is oxidized to sulfate without the production of any detectable intermediates. The indirect mechanism assumes that the cell or bacteria do not act directly on the sulfide-mineral surface, but catalyze reactions proximal to the mineral surface. The products of these bacterially catalyzed reactions act on the mineral surfaces to promote oxidation of the dissolved Fe(II) and S° that are generated via chemical oxidative processes. Ferrous iron and S°, present at the mineral surface, are biologically oxidized to Fe(III) and sulfate. Physical attachment is not required for the bacterial catalysis to occur. The resulting catalysis promotes chemical oxidation of the sulfide-mineral surface, perpetuating the sulfide oxidation process (Figure 1). 

Figure 1 Comparison of the microbial direct versus indirect oxidation mechanism for sulfide (source Sand et al., 2001).

The oxidation mechanism changes, influenced by the nature and location of substituents therefore, no single proposal is adequate to explain all results of the electrochemical experiments. For the alkyl aryl sulfides (XVI), the reaction pathways to be followed depend on the type of R group. With R = Me or Et the reaction scheme of Eq. (48) is suggested, but for R = i-Pr or PhCEl2 additional pathways may be considered [Eq. (49)]. 

Based on the results of the electrochemical experiments performed with the sulfides XVII, a reasonable proposal for their oxidation mechanism seems to be as follows ... 

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