Aerobic oxidation of sulfides

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]. 

Ishi reported on aerobic oxidation of sulfides in the presence of N-hydroxyphtha-limide (NHPI) and alcohols [56]. The reaction works at atmospheric pressure of oxygen however, it requires 80-90 °C, and the selectivity for sulfoxide over sulfone is moderate ( 85-90%). 

The binary system Fe(N03)3-FeBr3 was used as an efficient catalytic system for the selective aerobic oxidation of sulfides to sulfoxides [57]. The reaction works with air... 

A method for mild and efficient aerobic oxidation of sulfides catalyzed by HAuCl4/AgN03 was reported by Hill [58]. The active catalyst is thought to be Au(III)Cl2N03(thioether). A very high selectivity for sulfoxide was observed in these oxidations and no sulfone was detected. Isotope labeling studies with H2 0 shows that water and not Og is the source of oxygen in the sulfoxide product. 

Murahashi has reported on an interesting flavin-catalyzed aerobic oxidation of sulfides to sulfoxides (Scheme 8.4) [59]. Flavin hydroperoxides can be generated from reaction of the lowest reduced form of the flavin (16) and molecular oxygen. These hydroperoxides (18) have been studied in stoichiometric oxidation of sulfides to sulfoxides by Bruice [42]. 

More recently, Jansen and coworkers [90] used 4-hydroxyacetophenone monooxygenase (HAPMO) for aerobic oxidation of sulfides. Interestingly, both PhSMe and p-MeC6H4SMe gave the corresponding sulfoxides in >99% ee, which should be compared with the 99 and 37% ee, respectively, obtained with CHMO [86]. The flavoenzyme HAPMO, which has been cloned [90, 91], is a promising biocatalyst for enantioselective oxidation of sulfides to sulfoxides. 

Lang X, Leow WR, Zhao J, Chen X (2015) Synergistic photocatalytic aerobic oxidation of sulfides and amines on Ti02 under visible-light irradiation. Chem Sci 6(2) 1075-1082... 

Mixtures of sodium sulfide and finely divided carbon exhibit an exotherm on exposure to air. As indicated by the low MRH value, this is probably not a direct interaction, but arises from co-promotion of aerobic oxidation of the individual components. 

Khenkin, A.M. and Neumann, R. (2000). Aerobic photochemical oxidation in meso-porous Ti-MCM-41 epoxidation of alkenes and oxidation of sulfides. Catal. Lett. 68(1-2), 109-111... 

The oxidation of sulfide to elementary sulfur (S) or sulfate (SO4-) may take place when aerobic conditions exist. If sulfide is produced in the deep part of a biofilm in a gravity sewer, it may be oxidized in an aerobic upper layer of the biofilm or in the water phase (Figure 6.2). The details of the oxidation are not well known and may be due to chemical and biological processes. The final step of this process is sulfate, although sulfur in an oxidation step of 0 may be temporarily generated. Oxidation of sulfide that is released into the sewer atmosphere will be dealt with in Section 6.2.6. 

Injection of air the oxygen in the injected air will prevent sulfate-reducing conditions in the sewer. The DO concentration in the wastewater establishes an aerobic upper layer in the biofilm, and sulfide produced in the deeper part of the biofilm or the deposits that may diffuse into the water phase will be oxidized (cf. Figure 6.2). The oxidation of sulfide will mainly proceed as a chemical process, although microbial oxidation may also take place (Chen and Morris, 1972). Factors that affect the oxidation rate of sulfide include pH, temperature and presence of catalysts, e.g., heavy metals. 

The integrated aerobic-anaerobic WATS model has changed this situation. As an example, it is possible to use the model in a gravity sewer with changing aerobic and anaerobic conditions. As previously stressed, a number of in-sewer processes still need to be dealt with. Examples are the anoxic transformations and the processes related to the extended sulfur cycle, particularly, the oxidation of sulfide and the emission of hydrogen sulfide into the sewer atmosphere, including its further oxidation at the sewer walls. Combined use of empirical and conceptual models is still needed. 

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. 

Reaction of the sandwich-type POM [(Fc(0H2)2)j(A-a-PW9034)2 9 with a colloidal suspension of silica/alumina nanopartides ((Si/A102)Cl) resulted in the production of a novel supported POM catalyst [146-148]. In this case, about 58 POM molecules per cationic silica/alumina nanoparticle were electrostatically stabilized on the surface. The aerobic oxidation of 2-chloroethyl ethyl sulfide (mustard simulant) to the corresponding harmless sulfoxide proceeded efficiently in the presence of the heterogeneous catalyst and the catalytic activity of the heterogeneous catalyst was much higher than that of the parent POM. In addition, this catalytic activity was much enhanced when binary cupric triflate and nitrate [Cu(OTf)2/Cu(N03)2 = 1.5] were also present [148],... 

Like the examples mentioned above, most examples of metabolic flux analysis by metabolite balancing have redox balances as a central constraint used in the determination of the flux distribution. However, the redox balance is, especially under aerobic conditions, subject to uncertainties which make it less suitable for estimation of the fluxes. Part of the reason for this is to be found in futile cycles, e. g., oxidation of sulfides to disulfides, where reductive power is needed to reduce the disulfides. The net result of this reaction is reduction of molecular oxygen to water, and oxidation of NADPH to NADP+. Since the consumption rate of oxygen of these specific reactions is impossible to measure, the result may be that the NADPH consumption is underestimated. This is in accordance with the finding that when the NADPH-producing reactions are estimated independently of the NADPH-consuming reactions, there is usually a large excess of NADPH that needs to be oxidized by reactions not included in the network, e. g., futile cycles [11-13]. 

There is considerable evidence that, in nature, bacterial sulfate reduction plays an important role in the formation of some deposits of elemental sulfur. Free sulfur is not, however, produced by sulfate-reducers per se and its formation depends, therefore, on chemical or biological oxidation of sulfide. Microorganisms capable of effecting the latter reaction are discussed in Chapters 6.1 and 6.3 while isotopic selectivities associated with this conversion are summarised in Table 6.4.2 (see p. 406). As discussed in Chapters 6.1 and 6.3 colourless sulfide-oxidising bacteria, e.g. Beggiatoa, and Thio-bacillus, inhabit aerobic zones of ponds, etc. while in the underlying anaerobic zones, where light can penetrate, photosynthetic oxidisers, such as Chro-matium and Chlorobium, are active. 

When molecular oxygen is the oxidizing agent, oxidation of sulfides and sulfur compounds by thiobacteria, which are all aerobic, can speed up oxidation rates by many orders of magnitude over the inorganic rates (Karamenko 1969). 

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