Sulphide-oxidizing activity

Oxidation of di-n-butyl sulphide with activated manganese dioxide in light petroleum gave di-n-butyl sulphoxide exclusively126. However, the reaction was very slow at room temperature. This reagent is also suitable for oxidation of diallyl sulphides although, after 76 h, diallyl sulphoxide was isolated in 13% yield only. 

In 1960, Montanari and Balenovic and their coworkers described independently the first asymmetric oxidation of sulfides with optically active peracids. However, the sulphoxides were formed in this asymmetric reaction (equation 130) with low optical purities, generally not higher than 10%. The extensive studies of Montanari and his group on peracid oxidation indicated that the chirality of the predominantly formed sulphoxide enantiomer depends on the absolute configuration of the peracid used. According to Montanari the stereoselectivity of the sulphide oxidation is determined by the balance between one transition state (a) and a more hindered transition state (b) in which the groups and at sulphur face the moderately and least hindered regions of the peracid,... 

Equation (10.4) relies on a knowledge of both the sulphide and oxide activities. However, S was not yet included in their database for oxides. They therefore utilised the approach of Reddy and Blander (1987, 1989) to relate o,a s lo Ihe S content and sulphide capacities could then be predicted through calculation of oa o alone. The approach yields the following expressions for sulphide capacities for basic and acid slags ... 

In simple adsorption from aqueous solution, Hg has features in contrast and in common with the base metals. The hydroxy-cation is the active species in the model for heavy-metal adsorption and this also appears to be true for Hg. However, in contrast with Cu, Pb and Zn, the adsorption is less efficient and is strongly inhibited by the formation of halide complexes, as has been shown by Forbes et al. (1974) (Fig. 12-1). These authors also demonstrate that the adsorption of Hg to goethite is effective at pH as low as 4, allowing it to be trapped subsequent to sulphide oxidation. Whilst many minerals in weathered rocks and soils may each adsorb Hg, the relative efficiency of the hydrous iron oxides (Andersson, 1979) implies that these phases will be the dominant host in most exploration samples. However, the soil organic matter is also of importance and, although the association with Hg has been described as adsorption, it seems more... 

The failure to detect anomalous Hg above these buried deposits can at least in part be related to features of the weathering environment. In some of the case histories it was determined that the present water table was above the depth of oxidation of sulphides with the result that active sulphide oxidation had ceased. In others it was apparent that the low levels of Hg restricted the development of a halo. 

There are, of course, several processes that can frustrate the detection of such expressions. It is important to consider the rate at which gas enters and leaves the soil air. Baver (1972) quotes several authors and estimates that there would have to be a complete renewal of soil air every hour to a depth of 20 cm in a normal cropped soil in order to maintain its usual average composition and microbiological activity. If mineral deposits are to have adequate expression in the soil air, sulphide oxidation must clearly influence its composition at a rate commensurate with such rapid aeration. 

To summarize, the isotopic and textural evidence collectively implies the activity in the Belingwe belt of a variety of prokaryotic processes (1) sulphate reduction and possibly photosynthetic sulphide oxidation (2) operation of rubisco both in cyanobacterial stromatolites (as expected) but also possibly in non-photosynthetic sulphur-bacterial mats (3) oxygenic photosynthesis (in stromatolites) (4) methanogenesis and methane oxidation. Most probably, other sulphur-based metabolic reactions (e.g. dissim-ilatory sulphate reduction) were also taking place. This complexity is consistent with the relative timing of the metabolic phylogeny deduced from rRNA studies (Woese 1987 Pace 1997). 

Klein, J. and Henning, K-D. (1984). Catalytic oxidation of hydrogen sulphide on activated carbons. Fuel, 63, 1064-7. 

Steric effects are not of major importance, as shown by the fact that f-butyl-substituted sulphides can be autoxidised in the presence of a catalyst. Rather, it is the inhibitory action of traces of f-butyl sulphoxide which is responsible for the overall lack of oxidation activity. This bears out the observed order of effectiveness of sulphide antioxidants added to oxidising hydrocarbons [97], where t-butyl substituents confer excellent antioxidant properties. 

Flotation has been used for more than 100 years to separate sulphides, oxides and other salts from ores, as well as to obtain phosphates, barite, chromite and other materials. Up to 90% of copper, lead, nickel, zinc are extracted using flotation in the USA [152 - 153]. In Russia, flotation is widely used to additionally obtain apatite, barite and phosphates. Flotation of iron oxides is not used in practise yet, but the number of experiments carried out in this direction is rather large. The main physicochemical principles of flotation have been discussed above [59 -74]. Here, only some practical problems will be discussed. In [153], requirements are pointed out which apply to three-phase flotation foams, and the main components of the process are defined, i.e. surfactant - collector surfactant - frother activator, depressants, colligend, gangue. The peculiarities of flotation and foam separation in batch and continuous modes are outlined as well as the structure and properties of the main types of flotation agents described. As surfaces of the majority of mineral particles are hydrophilic in nature, hydrophobisation of particles is necessary for a selective separation. 

As non-toxic chiral Fe complexes have recently been used as catalysts [118-120], increased knowledge of their structure-reactivity relationships becomes pertinent. X-band CW-EPR spectra of [Fe °Cl(l)], reported by Bryliakov et al. [121], were found to be typical of high-spin S = 5/2 Fe complexes with EID K, 0.15. Using this complex, the conversion and selectivity of the asymmetric sulphide oxidation reaction was investigated in a variety of solvents. In previous studies [122], the active site was proposed to be the [Fe =0(l)] species. However an alternative active species was proposed [121]. Oxo-ferryl 7i-cation radicals are expected to have typical S = 3/2 spectra with resonances at geff 4... 

FIGURE 5.11 Amount of hydrogen sulphide adsorbed from its mixture with air on different carbons. (1) activated carbon fiber, ACF (2) oxidized activated carbon fiber OCF, (3) oxidized activated carbon fiber impregnated with copper, (4) oxidized carbon fiber impregnated with iron. (After Svetlana, S., Kartel, N.T., and Tsyba, N.N., Carbon 02, Intern. Conf. on Carbon, Beijung, 2002, Paper PI 68 D. 063. With permission.)... 

Another example is provided by Simonneaux et al. with macroporous optically active polymers (not obtained through anodic oxidation) bearing chiral porphyrins. These systems showed efficient catalytic and enantioselective activity for sulphide oxidations and epoxidation reactions [70, 71]. 

The tlrermodynamic activity of nickel in the nickel oxide layer varies from unity in contact with tire metal phase, to 10 in contact with the gaseous atmosphere at 950 K. The sulphur partial pressure as S2(g) is of the order of 10 ° in the gas phase, and about 10 in nickel sulphide in contact with nickel. It therefore appears that the process involves tire uphill pumping of sulphur across this potential gradient. This cannot occur by the counter-migration of oxygen and sulphur since the mobile species in tire oxide is the nickel ion, and the diffusion coefficient aird solubility of sulphur in the oxide are both vety low. 

Fig. 7.71 An activity diagram showing the competing formation of sulphides and oxides on chromium. The XPS data (lower) show how sulphide replaced oxide as the surface anion when oxide samples were heated in the gas composition marked on the O-S diagram, implying that the boundary should be moved. (Reprinted with permission from Pergamon Press after...

The use of optically active peracids for asymmetric oxidation of sulphides will be discussed in Section III dealing with the synthesis of optically active sulphoxides. 

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